Because we have a dungeon finder to get groups faster and SE already knows we have a wide variety of skill levels in random groups. Adding in "specs" that are just useless to the role that they are supposed to fill would just be an annoyance to the community at large.

Essentially because the game is built around set number of 3 specific roles in each group, they need jobs that can specialize in those roles. Hybrids and snowflakes are always useless in groups in the formula they are following for this game.

Even in FFXI there was customization but realistically if you wanted a good group you had only a set amount of choices per job.