Back in the wayback, glasses were a luxury item; only needed glasses if you were reading and you weren't reading unless you were either landed gentry who was sent to school or a very lucky/intelligent child of a commoner who got sent to school; either way, glasses got associated with intelligence back then because only intellectuals or "skilled professionals" who needed to do a lot of reading would need glasses; you didn't particularly need good eyesight to be a farmer or factory worker, but did need it if you were in a "smart" profession, so only the smart got them and it became an indicator not that all smart people wore glasses, but glasses were only worn by smart people. It hasn't been true for a few generations now but the perception/cultural shorthand hasn't changed quite yet.