Well, that's an easy answer. Politicians use attack ads because they work.
They work because of our gross ignorance. We like people because they're funny, attractive and can deliver zany one-liners. Attack ads remind us who these people really are.
"Oh, he killed a guy? Wow, I'll guess I'll vote for the other one."
These ads usually remind us about voting record hypocrisy that reveal a person's true self. Most importantly, they work.