You say that as if end game is the only content available in the game. Many people enjoy lvling process and don't want to rush to end game. Games with many expansions usually lower xp needed to lvl up or raise xp gained making lvls that where hard to get incredibly easy.
When WoW launched it took quite a bit of time to get to 60 unless you played Hunter/Warlock and just grinded mobs nonstop while today someone can get from 1 to 60 in few hours if they want.
I'm not saying endgame is the only content. however, in any "theme park MMO" it is clearly the focus when it comes to content - these games live and die through their ability to engage players with a regularly updated endgame.
nobody is saying you can't or shouldn't enjoy the leveling process (all the better if you do, honestly), but the way it is structured in so many games makes it unlikely that you will enjoy multiple reruns of it. heck, I don't conceptually dislike the leveling phase in MMORPGs, even if I do view it as more or less an arbitrary obstacle - I find it important to let the player get used to the game and their class at a pace that enables them to actually take in all that information. what I do personally have a problem with is having to perform tasks such as "kill x " or "collect y" over and over, especially if I want to try another class. I would most likely view the whole progress with much higher regard if the tasks involved were more engaging.