Can I ask why?

Not an optometrist myself, so please excuse if I'm using all the wrong words here. The way it's been explained to me: you want to leave some challenge for the eye to overcome, so the muscles that do the focussing stay in training. If you overadapt, those muscles have less to do and could atrophy slightly, leading to ever more loss of eyesight.

Sounds plausible, at least!

Can you provide evidence to this? Every new prescription I have gotten for glasses through my entire life has been too strong and required my eyes to basically degrade to be correct. Avoiding getting new prescriptions too often is pretty much the only reason my eyes are okay.

An optometrist cannot see through your eyes, so they use that lens comparing machine and ask you "which is better", and that leaves plenty of room for the patient to bias the results towards "different from normal is 'Better'", such that they vastly bias their new prescription upwards. Even if every optometrist is thinking about that and purposely downgrading the results of that flow, that could still provide room for the ratchet effect of eye prescriptions that many people with glasses have experienced.

> Can you provide evidence to this?

Wearing glasses for 40 years and talking to ophthalmologists and optometrists.

Also I don't know where you are getting your glasses from, but where I am it's been decades now that the measuring of eyesight (with a machine) is done before the lens fitting, which mainly focuses on astigmatism.