Artifacts leak. Specifically, while they recharge like a living mage does, in the presence of large numbers of people this magic will leak out into others. Leakage rate is a function of population density within a known distance.

This means artifacts can be studied and used in isolated universities, but in a war setting just having people nearby makes them useless.

Non-sentient magic can be disrupted by enemy mages

Your magic system could allow mages to disrupt magic unless the caster resists them. Spells might be magic constructs ("balls of light") which have to travel to the destination before they activate. Left uncontested, an enemy wizards might be able to disrupt a spell before it activates, causing it to dissipate without effect. If a wizard casts a spell, they could be able to resist enemy wizards trying to disrupt their spell, so that it activates as normal. But it could be impossible to defend a spell cast by someone else, or by an artifact, so that they are useless in war.

(You would have to say that continuous-effect magic like shields can be disrupted unless the caster actively defends them. Possibly there are disruption-resistant materials which would allow for permanently enchanted weapons, but the process of enchanting them is slow and creates magical "ripples" which allow enemy agents to detect and , and can be disrupted like any other spell.)

Magic is often a strangely misrepresented thing in fantasy. It's almost solely aimed at combat and war, which is not how it would really go if magic were "real".

Magic would follow the same lines as technology. There won't be any fireballs thrown in the beginning. Instead the first magic that was discovered is about the 3 S's: Sex, Shelter, Sustenance.

Sex contains everything from making sure you look good to attract a partner, convincing said partner, getting pregnant, giving birth to a healthy baby and then continues with getting the child to grow up healthy and strong while teaching him things he needs to know.

Shelter contains everything from building something against the elements, from a small fire to keep you warm to the clothes you wear (or don't need to wear because magic) and everything around your home. Hell even just something to prevent your home from catching fire would be worth gold in ancient times.

Sustenance is all about the food. Finding it, making it, hunting it, protecting it, herding it, growing it, keeping it free from disease and poisons, preparing it, stockpiling it, keeping it fresh etc. It is likely that you would see magical refridgerators before you would see advanced weapons.

The items your mages leave behind are focused on this, and likely given to their children or those they care about. An artifact that can cure diseased crops, keep the soil fertile or even just harvest all the grains without harming the stalks so they can be harvested again would be invaluable.

An artifact that can cure disfigurements and change one's appearance to their wishes would be a most powerful tool in such times (it would be a powerful tool today!). Or what about a tool that makes people immune to certain diseases? Child death would go down, horrible diseases that we get shots for today would be eliminated (but what if the artifact causes autism? GASP!). Even if it can only be used for a single family, a powerful mage would not hesitate to give this to his own family so they can be strong and healthy for years to come.

Or imagine leaving something to improve the homes of your family? They could be fireproof, always a comfortable temperature without the need for heating or cooling, the space inside could be larger, furniture could be immune to wear-and-tear. It might not seem that powerful, but if you never have to replace your furniture in the olden times it save a lot of time, money and effort. Not to mention how it can increase the status of the family.

Most days aren't about warfare, and as miss Nightingale proved even war is less about the war and more about the hygiene needed so infections dont kill literal magnitudes more than the actual fighting does. Many wars never even happened because supplies are more important, and the ability to send home your troops so they can help with harvest is invaluable for the country as a whole. The amount of magical artifacts focused on warfare would be minuscule compared to the amount of magical artifacts focussed on just making it through to the next year.

Usefulness besides war

It's been alluded to and outright said in some answers, but here's a deeper view on the usefulness outside of war.

Powerful artefacts can be used for much more than war. They might be things like a weather staff. Capable of irritation through rain or prevent fields from being flooded through sun these things can be used in war, but it's impractical. A large army can consume fields just as well, and large armies are probably required to defend such a piece in war. It would be too valuable not to defend it with a large army. That makes it not as effective and hazardous to your own troops.

These artefacts are a lifetime's work. There are too few applications for a lifetime's work to go into war. Politics, arts, agriculture, artisinal crafts and much more are the things where the work and money is. Sure some wizards will focus on war, but many will go to anything else. And the lifetime artefacts that are created by war wizards might just be altogether too dangerous to be used. Artefacts of destruction are seldom able to distinguish friend from foe, nor foe from useful things you might want to use later, like a mill of planting fields.

In short, artefacts are often more useful for prosperity. Lifetime artefacts used for war, or created for war, are too dangerous, too valuable and/or impractical to be used for war.

Proper application as weapons

We have lasers in industry that can cut a half-centimeter (approximately 0.2 inch) thick steel at a rate of 6m (~20 feet) per second. Fire it at a person's head and it's instant death.

So why don't we use it in war? Because it's a female dog to aim and shoot. It's a very heavy machine that takes too much electricity to power. It would have to be mounted on a nuclear powered tank. The tank would need to have technologies for cooling both the laser and the engine that have not been developed yet. And the target would have to be stationery, because the laser takes long to warm up and is only that damaging at its focal point.

Boeing and Lockheed were reportedly testing laser weapons mounted on military aircraft since the 2000's or so, but due to so many complications lasers still remain common weapons only in scifi and cyberpunk literature.

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With magic weapons it's the same. So yeah, Ponder Stibbon's Hex-controlled Reactor is a powerful device that is able to split thaums (the basic particle of magic). You could probably use it as a siege weapon against a castle, if you are able to spend a decade building the machinery inside the castle in the first place. The real world equivalent would be building the Large Hadron Collider around an enemy's bunker, and then tuning it so that particles collide at the bunker's walls.

Nuclear reactors VS hydrogen bombs:

When you think of nuclear war, you're thinking thermonuclear weapons. In reality, while nuclear bombs feature in the theory of war, the actual use for them is quite tiny. A nuclear weapon is too uncontrolled and terrifying to deploy, and if you did, then there would be an explosion (pardon the pun) of similar uses. Mutual assured destruction creates a strong incentive to not deploy such weapons.

HOWEVER, that doesn't mean nuclear power isn't going to war. Depleted uranium projectiles make tanks deadlier. Nuclear power plants power defense factories churning out weapons. Last but not least, all the really critical ships in the navy are nuclear powered.

The same principles can apply to your artifacts. Don't make a water elemental powered artifact swallow whole fleets, but maybe you can make ships that are almost frictionless in water. Don't make fire elementals that burn whole armies, but instead liquify metal and make armor for armies. Don't make the earth swallow enemy walls, but instead make stone-tipped arrows that go through metal like it wasn't there.

Direct deployment of artifacts is anathema, and will lead to uncontrolled proliferation and global destruction (who knows, maybe this has happened before, and your culture is built in the ruins of a once-great civilization that was destroyed by their artifacts). Subtle application of the powers of the devices IS, however, a perfectly acceptable use to benefit society directly and the military as a side-benefit. It all depends on how your magic system works.

Artifacts need to be atuned to the local properties of the magic field, a very slow and labour intensive process. If you really put your mind to it you can get one active in about 2 years of time, otherwise it'll have to stay within a few kilometers of were it was attuned to previously if you want to actually use it.

Problem of this answer: using artifacts for defensive siege warfare would be trivial, assuming that the artifacts effects are appropriate.

Artifacts are difficult to transport and their effect can not be easily directed.

Thermonuclear weapons are the most powerful weapons known to modern humans. However, the very first thermonuclear charges had a certain problem - they had to be built and detonated in one place. They depended on large amounts of liquid deuterium, and there was no practical way of delivering this weapon in form of a "hydrogen bomb". How good is the weapon if you can't strike your enemy with it? Only later thermonuclear weapons had become as portable as fission ones.

So, these magic artifacts just can't be brought into battle. Even if they are small (and technically can be moved), their activation process requires them to stay in one place for some time. They can only be used for demolition - for example, an enemy captures a castle, and defendants obliterate the castle, the enemy and themselves in one big blast - but this provides only a very limited use.