Earlier quoted context omitted.
Your vision could change faster than however often you need your cornea examined or whatever. Or someone may not be able to afford a doctor's visit but know they need glasses (ignoring the question of whether it's right for people to not be able to afford an eye doctor's visit). Extending your logic to other areas would end up with absurdities like needing a doctor to measure your waist size before you can buy clothe…
The impact of an improper prescription - headaches, eyestrain, creating lazy eyes - are far worse than buying pants which are too big.
Also, if someone feels they would get better vision correction working with a trained professional, I'm fine with that. I only object to _requiring_ a doctor's prescription to get glasses.