People would then complain that they have to farm items to get FATEs to drop.

Also they would have to change the name because it certainly wouldn't be a Full Active Time Event.

Moving on...

Seeing how people seem to really hate FATEs, but their purpose was to make things actually happen in the world-

"THEN WHY THE ($*@ DON'T JUST ADD NM AND HNM YOU STUPID MUTH-"

Whoa there. Calm down. They wouldn't do much with actually making things happen in the world because they would spawn, then they would die, and the "active things" happening in the world would be people standing around X Y or Z and waiting for them to show up.

"THEN WHY NOT A POP-ABLE ITEM YOU DUMBA-"

Seriously, calm down. A pop item would be fine. More power to it. Hell they have that in game now I can't for the life of me understand why they don't use it more, unless of course Japan really really loves FATEs.

Moving on...

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.