There’s a bunch of closed source ones, the best of which were brought by Adobe, but there’s never really been a big open source effort.
GIMP say that the lack of separation between functionality and UI is why they’re not behind an effort. That said they said they’d be happy to stick their name on an effort.
But this takes me back to my original question, why are there no open source photo editors, despite their popularity?
Most people I know, including me, edit their photos with Google photos. The learning curve is just do much better.