Question

Why do teens don't wear glasses when they have weak eyesight

Answer 1

they want to be fashionable and don't want to be called nerdy

Answer 2

The teenagers don't wear glasses when they have weak eyesight. This  is because they think that by wearing glasses their beauty is affected.
They think that by wearing glasses they are becoming less trendy .
WE cab convince them to wear glasses by telling them about the disadvantages of not wearing it like poorer eyesight , not able to see  things written on blackboard.