This was argued heavily when 2.0 launched. Yoshida does not want people leveling up doing things that are "Not Content" which were defined as Fates, Quest, and Dungeons. Aka this is a themepark you must do what the devs want you to in this game.

It would be a simple fix to make xp partys a thing, just raise the XP per mob kill cap, keep it the same as it is now for mobs = to you or slightly higher but increase it on mobs 7+levels higher up to a max of 800-1200xp per kill per player on mobs say 15-20+ levels above you and partys would be able to kill them for xp.

There is so much resistance in this and I really cannot understand how having this as an option is a bad thing even until this day. It promotes grouping and community... a hell of a lot more then fates which I feel are disliked by about the same majority of the community that dislikes party based leveling.