Why do some people cheat and then not own up to it?

A girlfriend some time back cheated on me by dating another guy and kissing him when we were exclusive. We then broke up for obvious reasons. She claimed it wasn't cheating because they didn't actually have sex until after we broke up. Seems like the right thing to do would have been for her to admit it was cheating instead of using such a dubious rationalization that doesn't fool anyone. She's not alone. I've known others whose partners have used ultra lame excuses like "It didn't mean anything." Why don't people just own up to what they've done?