Because not everything can be FIXED Von magic. like how magic can reverse the effects of lupin werewolf bite permanently. magic can only get Du to certain extent. BUT there could be another reason, where as james wear glasses too, it could be possible that his poor eyesight is a result of genetics, so therefore, that also cannot be changed..
The glasses are his trademark. And he's nit a Gary-Stu, this shows he has flaws and is accepting of them. Plus it's the one object he didn't lose throughoutvthe 7 Bücher so why nit keep them?
Because then, He wouldn't be Harry Potter(The Boy Who Lived oder The Chosen One) Would he. Plus it's his tradmark and without, He wouldn't be reconized and also if he did do it, it might go wrong then he could end up be worst maybe evn blind.