It turns out that when the dev team dumbs down a role, dummies make up more of the role's population. Dummies are attracted to the role because it becomes a ticket to getting carried through fights, and skilled players abandon the role because they don't want to be carried. The resulting poor performance is then used by dummies and non-healers to clamor for further dumbing down in a never-ending ouroboros of stupidity.
Partly, it's because the job design team is the same four people who started in ARR, even though we now have twice the number of jobs in the game.
Partly, it's because CBU3 has designed themselves into a corner, by warping the entire game around two-minute bursts (on offense) and one-shots (on defense). If you look at WoW, you can see that there are many more things healers can do, if you have fights that aren't just Dance Dance Revolution in disguise.
Partly, it's because every game has a finite design space, and if you keep adding new things to jobs, you're eventually going to run out of space. Honestly, I think that the approach to class/job design in MMOs should be to stop changing the job once it's in a good place. Then, if players want to try new things, that's what new jobs are for. You don't need to keep adding still more finishers and bloat, or removing job identity and homogenizing the jobs, just to give players an illusion that VerThisIsTheLastRdmFinisherWeSwearForRealThisTime changes their play in any meaningful way.