Back in 2019 they mentioned that "Krita has gone from 2 to 4 full-time developers over the last 10 months, and Steam has been instrumental in raising the funds to make that possible" https://old.reddit.com/r/linux/comments/d2ic2e/krita_is_now_...
I wish more project would do the same. I know the stores take a cut but my guess would be that it's easier to make people buy there than make them a direct donation for example.