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?
Krita is available, it may not be great yet as far as mobile friendliness in the interface, but I see they always do work a bit towards that in their release notes so hopefully it can grow to become the best one, it won’t be as good as an actual photo editor maybe, but it’ll be the closest we can get in the foreseeable future.
For simple edits there is still Simple Gallery Pro