Why don't people ride cows?

I saw on the news this morning that a hot German babe was riding a cow, and it was doing some sweet jumps over the rails and stuff. I know people ride horses, but why isn't cow riding as popular?
Update: No I live on a farm. Cows are quite quick. I'd like to see you outrun a speeding black and white bullet, aka a charging holstein.
4 answers 4