Why can't happy people make great art? Do we also think that sober people can't write great novels, or people who survive past age 27 can't make great music? I can think of countless counterexamples to all of these. Just because there were miserable great artists in the past doesn't necessarily mean you have to be miserable to be a great artist.
This is kinda just a myth as far as I can tell. There is a often romanticized relationship between art and suffering, because sure, some art is created as a way to relieve suffering. But it's by no means mandatory, and at one point I've even been in the spot (like many many others) of being afraid to get help for mental health issues because of what it might do to the work I was creating. This is how the myth does harm. The longer people "trade" health for art, the more they put themselves at risk in service of this misguided idea. It's no exaggeration to say that people have died as a result of believing that treating their depression would diminish their art. A person in that situation has to win every day of their lives, while depression only has to win once.
Happy people don't make great art.