Science is starting show that mental illness is a result of altered brain structure, so it's not a stretch to think that creative people see things from a different vantage because of similar structural differences. Overlap occurs so the stereotype persists despite evidence against it being a universal constant that artists are inherently unhappy.

Anyway, I've written a bit about mental illness and the brain if you want to take a look.

https://darhtbjehl.medium.com/every-molehill-is-an-everest-a-brief-primer-on-depression-and-the-brain-fdc0291b5f31