To put it nicely. Low to mid level content is put in games to grab people interest at the start. Endgame is put in to keep people's interest when they reach cap and prolong the game. The goal of a MMO is to keep people playing as long as possible. If they just made a bunch of low/mid level content and less endgame.

What is the point of even leveling to cap?
What do you do after you hit cap?

It's needed yes. But needed more then endgame? no. You need just enough to keep people interested until they reach cap. Real simple logic. If there is nothing to do once you reach the cap. People will lose interest and quit the game. You seriously expect a game to last 5+ years with no content for high level? and a bunch for low level? Low/mid level content is much better for Single player games where there is an end after you reach cap. Not for a MMO where you try to keep people playing as long as possible to keep getting money.