I think, in a nutshell, the problem is that if we actually argued with our preconceptions, said preconceptions would be blown out of the water and we'd all have to rethink the way we go about thinking about just about everyone.
And people don't like to change.
"So if you go to Washington, it's buildings clean and nice. Bring a pack of matches...and we'll burn the White House twice!"
"Nobody's talking about extermination. No one ever does. They just do it." - Magneto
"Don't solicit for your sister, that's not nice, unless you get a good percentage of her price."