Yeah you're right, it's kinda surprising that they can regrow an arm over night but can't fix poor eyesight. I was talking specifically about Harry seeing as he's the main character.

Edit: I just want to emphasise that I fully agree with OP and I'm specifically talking about why Harry has bad eyesight and not excusing why countless others have bad eyesight.

I hate to be a Harry Potter nerd, but canonically Harry's bad eyesight was caused by the same thing that caused his scar. We know because when Harry dies in book 7 and goes to King's Cross with dumbledore, he no longer has a scar or bad eyesight. That's because when you go to the afterlife in Harry Potter, only your soul is moved on and not damages that have happened to your physical being in life. Before anyone mentions it, ghosts, like nearly headless nick, still have injuries gained in life because they chose to stay on the earth to be as close to life as they can be, which would naturally include damage to their physical self.