It's not exactly something that FF especially delves into, so it's hardly surprising it flopped. FF doesn't do "build a job". At best you just generally get to take two different job skillsets and use them at the same time, that's... not at all like what you're talking about. It also would be appalling from a balance standpoint. That Elder Scrolls Online, when "build a job" is much more a key feature of that series, doesn't do that should say something about the viability of build a job in terms of an MMO.

In short, it's not really an FF thing or an MMO thing. It's hardly surprising that they went a different route.