Why don't my wipers clean the windshield very well?

If there is any kind of divot or crack on the windshield, even brand new wipers will leave streaks since the windshield could be off balance or the crack could be cutting into the surface of the wiper creating an uneven surface.

If you replace the blades and it makes no difference, it could be a worn spring in the wiper arm(s). Especially on an older vehicle or arm.

The windshield could have a bad depression spot in some area. Check with a metal straight ruler and try to pass a piece of paper under. A Sure Wipe spring added to the wiper arm will increase the pressure on the wiper and will improve the situation but if the depression on the windshield is too pronounced, it will only reduce it.

Besides worn wipers (consumables), being too greedy with the washer fluid so there's sand and grit stuck on glass/rubber, and physical damage to the windshield - here are the common ones:

Wiper arm is bent, or "bent back" incorrectly

Wiper arm spring has lost tension (more common in snow-winter regions when routinely leaving the wiper arms elevated overnight)

Incorrect / poorly fitting size / shape adapter installed when using aftermarket wipers, causing slop and incorrect angle

Wiper arm linkage / axle bushings are severely worn out, causing the arm and thus the wiper blade to tilt over at too big an angle (from the windshield) as it moves

Driving with the window heating ("defogger") on. It is causing the wiper rubber strip to thermally expand and contract unevenly along is lenght. Do this a lot and the deformation becomes permanent.

If you have a moisture problem (or dirty glass) in your car, odds are you're driving with the heater blowing up on the windshield all the time to keep the fogging away. Most cars, especially somewhat older ones, have heat coming up on the window unevenly. If your window is frozen over you'll see the hot spots when the heating first start tawing the ice.These spots creates very different hot and cold spots along the "line" where your wiper comes to rest. That causes the wiper's rubber blade to expand and contract unevenly along its length, making it slack in spots, only dragging along the water instead of wiping it away as it will do in the areas of the rubber that is less thermally expanded, and thus it stays firm. If you drive like this a lot, this uneven expansion of the rubber will become permanent deformation.

Solution: Clean your windows inside with an effective degreaser, do it over until you can drag your finger across (in a corner) and get that 100% clean glass squeeky sound. Yuu can also blow on it, see how fast the fog dissapates - it should be really fast on clean glass. And sort out any moisture problem so that you don't have to drive with the window heating constantly on to keep the fog away. Only use it when you absolutely have to, otherwise do not heat the windshield when driving in cold weather. Allow the glass to stay cool, closer to the temperature outside, and it won't fog over so badly and snow won't stick so easily to it. You can still heat the car, just refrain from heating the window directly unless you absolutely have to. If your car is so equipped, try using the heated windshield function (most common on Fords) instead of the blower.

And get new wipers. If the wipers blades are slack in spots when cold, the damage is permanent. No point in buying expensive ones, as it's not wear "killing" them. Stick with cheap ones and replace often, until you get the fogging sorted.