I think there are a couple of factors at play here. The first is that you never hear about how normal an artist was. Nobody talks about how Ansel Adams didn't beat his wife and wasn't an alcoholic, because that's just normal. We only talk about Hemingway's alcoholism, or Van Gogh's mental condition. So we're conditioned to believe that artists are flawed people.
The second factor is that people want to justify why they're not a great artist. "I'm not a great writer, but at least I don't emotionally abuse everyone I know and I'm not a raging alcoholic".