From what I understand the long time wait for the new quest FATEs to spawn is a bug - they're actually not supposed to take that long to spawn apparently.

Having said that, I think the whole reason SE went this path is simplicity - rather than have dedicated instanced fights for players to do these quests, which SE knows would be congested at launch of the patch, (after all, remember all the trouble players had doing the early story instances in phase 4/early access, where players were literally lining up to enter the instance?), with a FATE anyone can participate and as long as they do something in the fight, they get credit for it.

It's also far easier to code a new FATE with a single mob repeatedly spawning into the game instead of a special instanced fight. So it's simple pragmatism here.