Why are music videos on YouTube so poorly synched?

I love digging up music videos on YouTube. But I’ve noticed that with the vast majority of them, the artists lip movement and singing are horribly out-of-sync. Is this a technical problem with YouTube or the posters? Are people trying to dub music from their computer to silent video and missing the mark?

Oh, and letting users tag videos sucks. You get absolute garbage in searches. Oh, well, you get what you pay for.