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?
Likely because editing on desktop is much more common and so gets the majority of the focus. A comprehensive photo editor for a mobile platform is probably just too niche to attract enough developer interest
Amateur editing on a phone probably out weighs desktop by orders of magnitudes. Most people don’t use desktops outside of work and education.
Unfortuantely, how I see it, FOSS devs are more likely to cater pro/enthusiast users who are more open to try and support desktop solutions. Meanwhile, indie Android devs are also likely to monetize their work. I can’t blame them, though.
Most people I know, including me, edit their photos with Google photos. The learning curve is just do much better.