It's simple. The devs forgot that the 5 roles are supposed to approach the game in 5 very different ways, even if they softly overlap with each other. While there has been homogenization issues dating back to ARR, the second the devs forced everyone to do every mechanic the same way, which started being the norm in mid-ShB, the devs lost the ability to have both diverse class and encounter design.

This in turn forces them to homogenize classes that can't do the ever-stricter encounter design, which in turn dilutes any differences the classes have both between roles and each other. Compound this with a desire to make every class have something 'new' while forgetting to prune old stuff in each expansion and you get situations like BLM's leveling experience or GNB getting a third cartridge for some reason as the devs slowly draw the class further from its relatively perfect launch state.

It's like the devs are stuck in some sort of demented loop where the only solution is dragging the game more and more into the extremes, and people either love the extremes, or leave.