I can't deny that I'm also somewhat annoyed by this situation: I have most of my jobs at 50 so I have a good start, however anything beneath that is a pain to level because it's pretty much depending on the amount of people nearby.

Sure, I CAN do dungeons but after the daily exp bonus all the other runs aren't worth the 10-30 minute queue as a dps. For a tank and healer is a different story, but not everyone is a tank or healer.
Sure, I CAN I do quests but they run out almost immediately and they don't refresh when switching jobs (which would have helped IMMENSIVELY).
Sure, I CAN do leves but they also run out and the time it takes to finish them, you might as well do fates

So in the end you want to do fates mainly because you get a boost (if you're levelling a secondary class), they're usually quicker (with some exceptions, but some fates took me less than 30 seconds to end), and they actually respawn from time to time (unlike quests, that don't refresh after switching class). Yeah there are other ways, but FATES are essentially the only viable way to level up in this game. And fates are a lots of fun right?!

...R-right?