Why can't HSS mode be automatically turned on for shutter speeds faster than the max speed sync?

A high-speed sync mode is in certain manner equivalent to using a neutral density filter.

Here's why.

If you use ND4 filter, you get one fourth of the light through the filter in front of the lens. This reduces all aspects of exposure, both flash exposure and ambient exposure to one fourth of what they used to be.

If you use high-speed sync with a 4 times faster shutter speed, you get due to the fast shutter speed one fourth of the ambient light. However, if the power supply of the flash is capable of delivering a certain amount of charge through the flash tube, all light produced by the flash expose only one fourth of the picture. To expose the entire picture, you essentially need four times the charge. But the amount of charge that can be put through the flash tube is limited, so you get approximately one fourth of the flash exposure as well.

To me, it seems like a useful feature to limit flash shutter speed to the sync speed, because that's the speed after which the effective power of the flash becomes to diminish. You want to know you get diminished output, just like you know when having to put an ND filter in front of the lens. And it's not a big deal to turn the HSS mode on to allow fast shutter speeds.

Of course it would be a simple firmware change to allow using HSS mode without having to turn it manually on.

Nikon does.

Nikon speedlights automatically switch into HSS, and Godox strobes do as well with the correct TTL radio trigger. You just have to set the flash sync to 1/250* (or 1/320*) in the camera's menu. The star indicates "auto FP" which is focal plane shutter sync (HSS in Nikon language). I'm pretty sure others do/can as well.

But there is often a problem in the transition shutter speed range. It's not the actual shutter speed that matters, it's the length of time the sensor is fully uncovered - which is the shutter speed minus the shutter travel time. That's typically 1/300sec or so, meaning that in practice the x-sync window is roughly 1/500sec (at 1/200-250sec shutter speed). That mismatch commonly causes slight darkening/falloff gradient along the edge of the frame when shooting at max x-sync speed.

Often the edge gradation/falloff/loss of light isn't noticeable when photographing outdoors or when the light isn't the primary source, but it's usually there; and it can be a problem. That's one reason why it is usually preferable to use a slower shutter/sync speed than the max possible.

Of course, they could then program the switch to HSS to be earlier as well, but often the negative of the timing mismatch/gradient is much less of a problem than the loss of effective power switching into HSS is. So, ideally they should give you a lot more options... but then everyone would have to know why you have so many options and when to use them.

And in reality, it's usually just easier to do it all manually if you do know what/why/when.