General argument: "You don't actually have to participate in FATEs if you don't want to."
This is unfair towards those who enjoy other forms of leveling up. Since FATEs are the single most powerful and easy way to level up, efforts for other, more challenging and to some more fun ways to level up will not be rewarded enough in comparison.

General argument: "Then why not just buff everything else up?"
1) This would still keep large crowd FATEs Whack-a-Mole competitions with no proper class roles and no challenge beyond said competition for XP.
2) Nerfing one thing is easier than buffing five.
3) Turn-in FATEs would remain abused.
4) (Opinion/personal) Too fast level-up speed will drain the contents of the game dry too fast. Not good for long-term commitment and renewing subscriptions.