Because they try to draw without practice and then post it on places for glory. They get terrible comments instead. With a bit of practice, they find that there are still terrible comments, but more constructive criticism. Unfortunately, they don't listen to constructive criticism. They continue to be terrible and refuse to believe that actually practicing will yield results, and frustrated after their results do not meet their expectations, they decide to steal another's work. Most of the time, they try and find something obscure, but obviously isn't, and get sniped. Even if they do find something obscure, there are always obscure people that will find it and they get sniped. Unfortunately, people have figured out that sometimes stealing work from the get-go actually works, so they stick with it. Also, if someone actually does find them stealing after a while of them getting away with it, they spew out random excuses that make no refutable sense.
sry rambl'n