Who knows? It may be just insecurities, maybe by telling you their work is awful then you may be more forgiving. It can also be that artists know their weaknesses better than anyone, and may be under the false belief that others notice in their art every little detail they're convinced is not good enough.
I'm a very harsh critic of my own works, people often get the impression that I hate everything I do, but I don't, I just try hard to find everything that could have been better, so I can learn from it and improve my craft. When I read my finished works (in comics it may be a while, even years, between me finishing a script and an artist illustrating it) I'm always thinking how would I make it better if I was writing this today.