superman

It's more along the lines of mannerism. Superman often stood tall, spoke with confidence and authority and more or less was able to carry himself as a "go to" guy for the problems that were being faced. While as Clark Kent, he often slouched, wore extra baggy clothing the cover his build, spoke timidly(with a change of voice in itself due to percise vocular muscle control), and tried to make himself look as weak as possible(though this has changed somewhat lately). The glasses were simply and addition to the whole persona of Clark Kent to throw people off. This wasn't really pointed out, however, until the mid to late 80's, I guess in response to the posed question of this topic. While the persona of Clark Kent has evolved in comics as of late, Clark not really being anyone's pushover as he once was, it still boils down to mannerisms and differences that Superman portrays as Clark Kent to more or less don't even have people even trying to compare him to Superman to pick out differences or similarities between the two.

It's a part of story-telling, it's a style. If he can be recognize when he wears glasses then everybody will know he is superman which kinda spoil everything. There must be an element of surprise in creating stories.

because his glasses have a magical darkness that covers his features from being seen