Why is it that liberals think all conservatives are "stupid" but conservatives don't think the same of them?

I've heard a lot of liberals saying "if you're a conservative you must be stupid." Yet conservatives do not make such generalizations about the other side. I am a conservative and I know that there are smart liberals (smart but misguided maybe even crazy) and can admit that they are intelligent. Liberals think that we are all "rednecks," which is just so untrue. First of all, most of the country is rural and conservative people do live in large cities, like myself. Liberals like to make sweeping generalizations about conservatives (redneck, religious fanatics, ignorant, uneducated, stupid) Basically we are all troglodytes to them, even though we make up most of the country. What's up with that?