Yes and no. XI's RDM easily proved that in a game where everyone is efficiency-driven, you can't afford to be a generalist. Has nothing to do with playing properly, and all about exploiting what little you could with that particular job. Curing and buffs had no stat penalties the way melee and nuking did. That's why people used RDM the way they did.
Conversely, WoW's Paladin proved that you can have a hybrid that works, but you need to put the time to give the class mechanics that make its multiple facets interact. Of course, that also means you have to help them lean towards a particular role chosen by the player to make that transition easier to bear.
Rather than why, you should be asking "why not?". Game mechanics, class balance, inventory limitations and stat scaling are good reasons why the focus on the favorite makes sense.When presented with a class that can melee, heal, and dish out magic dmg, the usual choice is to focus on your "favorite". Why? I have no idea.
I don't know how much experience you have playing hybrids, and how much ire, discrimination and ridicule you've had to stand (from both the playerbase AND the developers) for liking a class or job that is composed of several aspects, but RDM and Paladin's treatment in their respective games shaped my view on hybrids. I've yet to see another model for hybrids that works and makes them part of the rule rather than the exception, so...