Just look, for instance, at FPGAs: almost all the tooling is proprietary, very expensive, and very buggy too. Or look at PCB design: Altium seems to be the standard here still, despite Kicad having made huge advances and by most accounts being as good or even better. It took decades (Kicad started in 1992) for the FOSS alternatives here to really catch on much, and only really because PCBs became cheap enough for hobbyists to design and construct their own (mainly because of Chinese PCB companies), and because CERN contributed some resources.
I'm not sure what the deal is with engineers hating collaboratively-developed and freely-available software, but it's a real thing in my experience. It's like someone told them that FOSS is "socialism" and they just reflexively dismiss or hate it.
Most of the incredibly well used robust open source packages are sponsored by large tech companies. The embedded space just hasn't had that kind of sponsorship.
Anyone who sincerely thinks GIMP can replace Photoshop or is otherwise good will never understand why professionals eschew open source software when there's work that needs doing.
The functionality wasn't even the biggest problem. And JFTR, I'm not talking about anything that came to Photoshop in the last... 15 years or so, I was just slicing PSDs for table layouts and making wallpapers, not actually editing photos like a pro :P
Well, it's pretty good for multi-monitor setup. Dedicating the whole monitor for the image without endless panels in the way is convenient.
The most likely reason is the GIMP devs don't consider it a use case.