My thoughts and suspicions:
Perhaps because many photographers are closet painters?
Why did Henri Cartier Bresson give up photography?
My theory:
Secret penis envy of painters, sculptors, etc.
Why is it that Henri Cartier Bresson gave up photography towards the end of his life and decided to pursue painting instead? Simple — he wanted to be a painter all along! But (unfortunately), he was too good at photo. This meant:
He became a prisoner of his own success.