Anecdotally, the reason for that is probably that myself and people I've spoken to don't identify themselves as a role. Rather, for example, they identify as a WHM that happens to be a healer, for example.
This is a bit personal too, as whenever I play SCH or AST, I don't really enjoy myself. I've been asked a couple of times to do it in pugs and statics and I hated it. Guess I just won't be able to pug if the pf strat calls for astro and sage to use their unique, unhomogenized tools, and I only enjoy white mage.
If SE wants to avoid having people feel like that, then I can see how this homogenization has come about. I wonder if they can avoid this situation, while also giving each job unique tools still... and how impactful can those tools be.