While I am not singing the praises of mob grinding as a game design model, I am going to use it for the sake of comparison here. When grinding mobs in older MMOs, you could head to a zone and start killing things solo. If you met up with people, you could start a group. Perhaps you started off in a duo. This allowed you to move to harder things. As you added more people, you could move to harder stuff or larger groups. If people had to leave, no big deal. You either replaced them with new people or just moved back to easier content. You could organically move from one thing to the next, everyone was always making equal progress, and it did not really matter how many people, or who specifically, were in your group.
In quest heavy advancement, you cannot do any of this. New people who show up are almost guaranteed to have totally different quests to do, and if your group shrinks below a certain threshold some quest chains will no longer be possible at all (so you will have to stop those chains before completing them for the big payoff at the end). This can be extremely frustrating and unfulfilling.
Then there are quests of the "gather 12 rat tail" variety - the ones where each rat either drops 1 tail, or only has a slim chance to drop 1 tail. Grouping for quests like this is a huge disadvantage, because adding one person doubles the time it will take to finish. Some newer MMOs have done a good job of eliminating this problem, and making sure when a quest item is dropped for one person it is dropped for all, but this is not always the case.
There are many things in MMOs that require suspension of disbelief. Spells, dragons, faster than light travel, teleportation, and other genre related features are things players have no trouble accepting. But it is incredibly jarring and immersion breaking to complete an epic, 20 step quest chain by returning a Princess to her father, just to see 2 more copies of the same Princess running along behind other players also finishing the same quest. In games where quests are an optional part of the game, or not the primary focus, at least you don't often have people solving the same quests at the same time. So by the time someone else shows up you are long gone. In quest heavy advancement games, you watch hundreds of other players solving the exact same problems for the exact same NPCs right along side you.