What do people see Vincent Crabbe and Gregory Goyle as? Imbeciles? Sycophantic yes men for Draco Malfoy?
Does no one see that Goyle reads poetry behind closed doors? Or the fact that Crabbe is an amateur chef?
No. And why? Because none of that is true. At all.
This is one time that judging a book by its cover does not give the wrong impression. Unfortunately they are usually just Malfoy's bodyguards, sounding boards, and the doers of his dirty work.
And if there were more, would people see it? No because no one gives the bodyguards a second glance.