Because hearing people either are truly ignorant (and need education about deafness), or because they are raised to make fun of that which they don't understand.
Most hearing people have had no contact whatsoever with anyone who's deaf, and they aren't taught much about the right thing to do with deaf people. Those of us who have deaf friends tend to understand, but those who don't rarely even know a culture exists around signing.
Those are just some things I've noticed.
By the way, I'm hearing, with some deaf friends.
