Well, the way I figure it is, lots of people notice that Clark Kent resembles Superman, but it doesn't occur to them that he is
Superman, because Superman doesn't wear a mask and thus isn't thought of as having a secret identity. So they just figure Clark is one of those people who happen to look like celebrities. He probably gets people coming up to him all the time and saying "Hey, anyone ever tell you you look kinda like Superman?"
See, that's the brilliance of the glasses. If it occurred to anyone that Superman had a secret identity, they'd assume he'd wear a wig and fake beard, maybe a fat suit, something complicated to really disguise his appearance. They'd never believe Superman would be dumb enough to think just a pair of glasses would work. And that's exactly why it does work!
I liked Birthright
's take on it -- that the feature of Superman's face that everyone notices most is the vivid blue of his eyes, and so by wearing glasses that damp down their shade, he changes the way people perceive his face.