Changing the UI would raise so much uproar. Millions of workflows would have to change.
In addition, it just does so much that changing it would be a very costly task likely to break a lot of things and end up removing features that some tiny niche still uses.
I didn't even really use Photoshop in a professional capacity, but it's probably worse in that situation. In the past, if you wanted to do something "cool looking" on the web, you'd describe to Google something you saw and invariably land on some Photoshop tutorial. It doesn't translate to other things easily. So to switch to something like GIMP, you have to not only learn the interface, but learn how to translate all of the useful help "in the wild" from Photoshop to GIMP.
I remember when I sat down to try to learn paths and such in Photoshop, thinking, "Is this really the best they could come up with?" But, yeah, it was ... when they came up with it and everyone learned how to use it. A switch would eliminate the moat that the product has.
And that's what's been chipped away at, in general. I haven't had any need of Photoshop for web dev work in a long time. My image processing needs rarely go beyond the command-line, these days, because many other things we relied on PS for are described in css or are vectors rather than image files.
In addition, it just does so much that changing it would be a very costly task likely to break a lot of things and end up removing features that some tiny niche still uses.