Why can't we have jobs with this level of depth and customization in FFXIV?

[Velox]

I really want a Support slot, that deals with buffs/defbuffs, damage mitigation and heals. Stuff like that

There is already a job like this, it's called Scholar.

Contrary to popular belief, dedicated "support" classes (as in, a class that does nothing other than buff/debuff) are not only the biggest pain in the ass to balance in any game environment, but they require an existing system to undergo thousands of dollars in alterations to accommodate said change.