(I have a friend who is firmly against repeating jokes and stories -- and in general, I am as well. But not this once. So if you've already heard me tell these, I sincerely apologize.)
"Ya know how I know Clark isn't Superman? Superman could write a 500 word story without passive voice."
"Superman would know not to bury the lede."
"A split infinitive?! Who do you think you are, Kent, Captain Kirk?"
I saw Man of Steel over the weekend. I have some very mixed feelings, but I didn't dislike it, which I suppose is good. It definitely got me in the spirit of Father's Day, let's just say that.
It's not a spoiler to say that at some point in the course of Clark Kent's life, he dons a pair of nerd glasses and joins the staff of The Daily Planet. I don't think it's too big of a stretch to say most fans find this disguise pathetic at best. (Though the good folks at DC did apparently once try to come up with a scientific explanation for the poor investigative skills of all the investigative journalists around Clark.)
I think the reason that people don't realize or assume that Clark Kent is Superman is pretty simple -- human beings believe what they want to believe. No one wants to believe that Clark Kent (the guy who screws up their coffee orders) is a demi-god. No one wants to believe that the shmuck who picked up the tab at happy hour is also faster than a speeding bullet.
Superman as a character is pretty perfect -- moral, altruistic, loyal, handsome, and all that jazz. But I think it's safe to assume that the Man from Krypton was also imperfect. He might not have tripped in the hallway, but I don't think he was any better at loading the toner into the printer than anybody else.
A character's flaws are not just necessary to make a character interesting -- they make that character believable. No one believes that Clark Kent is Superman because on some level, Superman is just not believable -- not even to the imaginary denizens of Metropolis.