I pose this Question back to you, look at the guys who fight in MMA, or perform in pro-wrestling.
Think about what they look like 'dressed for work', the taped ankles, and wrists, the shorts/spandex, the chiseled physique
Now, put them in a 3 piece suit, comb their hair, and put on a pair of Buddy Holly Glasses (or Drew Carey Glasses for those too young to know Buddy).
I'd lay even money, with those 3 steps they'd be next to impossible to pick out of crowd, add to that, that Superman acts totally different as Clark than when he's Superman
and that is why no one recognizes him out of costume.,its more than glasses, its attitude, and the full 'look', also by not wearing a mask as Supes, who in the DCU, would even assume he has a secret ID some who know Clark, have figured out he has a 'secret', but often as soon as they suspect some situation arises in which a double is present so that CK and Supes actually meet as far as any one can tell, thus dispelling the idea that they are the same person