Why did Harry Potter need glasses?

maybe he had bad eyesight

in fact

in the books

one of the potions that HP uses gives him good vision

(he didn't have to wear his glasses while the potion was in effect)

Of course

the glasses were a "character trait" that the author carefully selected for the hero

and so

of course no simple or safe or "good" spell could permanently cure the condition

else the hero would lose that trait.

Note that Dumbledore (arguably the most skilled of wizards) and McGonagall also wore glasses.

Clearly in that fictional "setting" there was no permanent cure for vision impairment.

If you pay attention to the intricate details of harry potter, nowhere does JK Rowling allow anything in her writing to fix human frailties, such as curing diseases or ailments, and I think the reason for that is some people cannot differentiate between fantasy and reality

That's exactly what I've been thinking since ages... Like you can fly you can be invisible but you can't fix your heckin eyes