If GIMP prioritized matching Photoshop's UX to make it easy to switch over (shortcuts, UI similarity, etc.), it'd be much more successful at converting Photoshop users. Serious Photoshop users at this point have spent more than a decade (!) committing these things to muscle memory.
GIMP hasn't made replicating Photoshop's UX a priority, so it isn't a replacement for someone more comfortable using the industry standard tool.
If I used Vim for a decade, then you said, "Notepad++ also edits text, why don't you switch over?" Well, I'm not as productive in Notepad++, and I don't think I'll ever be. Same for GIMP.