I know some artists and it's quite rare to see them finish stuff on time. Many just don't finish stuff at all.

Fact is that clients feel entitled to results, so they create pressure, which is the most counter productive thing you can do for creativity, since it needs to be free and open. Add to that a splash of mental health issues and perfectionism and there you go.

Artists also tend to be good at a lot of stuff so they are constantly asked by people to do stuff. They're constantly busy solving other people's problems. They're valuable people in society but often are quite miserable from stress and poor.

There's more to it than that but it gives you an idea why artists hate clients.