Most people are interested in the massively multiplayer aspect of MMOs. Not the endgame consisting of small groups farming dungeons repeatedly.
I don't get what's more massively multiplayer about leveling up versus doing dungoens.

You seem to be missing the point about MMOs, not everyone else. The game is just beginning when you reach the highest level. This is true of pretty much every MMO in existence. FFXIV even steps a little bit away from that for the first time player by giving them an engaging story to play as they level up the first time.

Leveling up is never the best part of an MMO. Which isn't to say it's not enjoyable, but it's just one piece of a very large pie.