FATE is no fun at all, especially if you build content upon it.

[Prototype909]

In short, FATEs exist because they wanted things to be happening in the world in real time, with events that come up and players can fight together without having to form a party. Something quick and easy and gets the job done. For this purpose, and despite people saying they hate hate hate them, they continue to see use. Should they stop using them for seasonal events and the like? Probably. But it is their method of getting people to play together, and they can't risk putting it in a trial or something and having DPS sitting out missing the event for hours at a time because lawl.

FATEs are like the bare minimum for content people can do which could be classified as "playing together".

It's less playing together and more playing solo among a horde of other solo players with whom you have no interaction beyond contributing damage together.