Artists aren’t obligated to do anything. If it’s an obligation then it isn’t art.
They aren’t obligated, but many leave out diversity almost by accident or out of habit. I think we should be careful about forcing an artist to do something they don’t feel is right for their story, but there should be someone there to check and ask the questions that need asking. Does this character really need to be a white man? Is there a good reason why 90% of the speaker characters are dudes?
Again, it’s not a matter of forcing, it’s a matter of looking long and hard at the work and asking why it isn’t diverse. The truth is, most of the time, there isn’t a good reason.
I suspect the reason is that people write about what they know and most writers old and experienced enough to be writing big movies or TV shows probably don’t “know” diversity so they don’t naturally write about it. Since most writers are men, they write men more. That’s my assumption.
If it’s a story about being black for example, I’d get why a white writer might not want to go there. But if they’re skin color isn’t that important to their story, there’s not a great reason why not.