This. In MMO, it is a never ending chain of jobs: what, you've finally beaten this heroic raid and got your gear? Guess what, you are going to have to do this 10 more times to get all the gear pieces. What, you've finally managed that as well? Guess what, there is the next raid, mechanically very similar to the previous one, and you are going to beat it 10 times as well, only to progress to the next raid...
...
What, you've finally beaten all raids, did all the achievements the game has to offer, got all the gier pieces you can possibly get, got all the mounts, etc. and the game has nothing more to offer? Guess what, there is the next expansion coming out in a week, and it will offer 10,000 more hours of chores - go grind some money before it fires up, so you can reduce those 10,000 hours to 9,000!
---
In single player games, in comparison, you clearly see the results of your actions. I am talking about games like Mass Effect or Witcher series, focused on the story, not on grinding like Diablo series. If you kill that boss, you don't have to fight it 10 more times - it will already be dead. Once you are done with the whole story and all the quests (rarely takes more than 100 hours in these games; well, in Witcher 3 it is probably more, but that's an exception), you've completed the game, and you are done with it, unless yo want to replay it once again.
What is 100 hours in an MMO? 4 days /played? It isn't even considered a beginning, just, well, exploring and checking out the game. I had 20 days /played in SWTOR, and I only went through 2 stories out of 8 (out of 16, if you consider playing light and dark characters to be different stories), I hadn't even touched operations ("raids"), only played a few battlegrounds and so on. 480 hours in - and I didn't feel like I made any progress.
I guess MMOs may be good for little kids who have tendency to find a lot of fun in pretty much everything they do, or people with too much spare time on their hands and few goals in life. I have a hard time imagining myself spending 5 hours a day for 5 years on something that doesn't give me much fun during the process of playing, and where the results are virtual "mounts" and other stuff which doesn't exist outside the game and doesn't even really matter in the game itself. Sure, I have probably over 2500 hours total invested in Starcraft 2 - but every minute there is fresh. And when I do get tired of it, I can just stop playing it for a couple of weeks - I don't have any "guilds" with "raiding schedules" or "dailies" which, if missed, will harm me significantly and slow down my "progress".
Bottom line: I play games that give me a large amount of fun throughout the entire process. MMOs feel like fun only when you are celebrating some achievement or getting new gear/mount, which is about 0.01% of the overall time. No, thanks. Even writing this post game me more positive emotions than MMOs give at the same amount of time.