I'm a designer that works both on an indie freelancer level and an agency/corporate level, so this feels apt for input!
Several reasons - you've got part of the answer right in that there are a ton of other smaller tools for the more specific sections of the market that can be broken apart and sectioned off. I personally deal with quite a bit of comics/illustration and there alone you have Corel Painter, OpenCanvas, PaintTool SAI, Manga Studio, Clip Studio, plus many of the newer ipad sketch programs that've popped up recently.
However - I think those markets get saturated and weakened too quickly by those newcomers that gain a foothold and then slowly start deteriorating due to lack of updates or shiny marketing. Photoshop gets underrated for sheer versatility in this case - it's less of a high priced transient brand (think Uber or BMW) and simply a functioning longstanding tool (a car, or a swiss army knife for that 'tool as a brand' metaphor) for so many kinds of design related issues that the ease of use is baked in. Software tends to have a maddening problem of quick degradation and a large slice of older designers tend to be very picky about their tools. Broke, too.
Additionally, for some of the larger corporations - there's lots of production chains that rely quite strongly on specific file formats to be exported or special color management, and most of these smaller programs simply can't handle that much depth without the team that Adobe has. (Don't ever talk to me about color management for factory packaging production.)
Finally, factor in the immense marketing team that Adobe has - designers are innately marketers, after all, and they've saturated most trendy design sites with pro-Adobe ads and likely have special deals behind the scene to keep their name relevant. Not a fan of that style of closed garden at all, so I skate by by using the most outdated version of PS (ie non subscription) I can find without crippling my workflow, and using others if I can.