That's because MMOs are designed like virtual theme parks. Quest lines are like a series of rides.
So true -- at least from my point of view. However, the massive success that these games have proves otherwise. Well, admittedly Star Wars and LOTRO are not truly blockbusters (despite their huge existing fanbases, which should guarantee a license to print money!), but I think that's because of the implementations that lack "design love".
MMORPG is already bad design by definition [...] The player want to be THE unique hero,
To be honest, WoW is leading the MMORPG into the wrong direction and most other MMORPGs follow
I've played LOTRO (at the time of Shadows of Angmar) myself and found it kind of entertaining for a week. However, overall, it was just badly designed in a purposely stupid, annoying manner. Plus, nothing interesting to be done after playing for a week, and nothing to be done at all (except raiding, which is really 90% waiting) after two weeks.
I remember reading a review when Rift came out (was it Rift? I think so...), of which the bottom line was that it's such a great game because after selecting the first quest NPC, you never even have to think any more. A big arrow shows you where you have to click next to "win".
All in all, my impression is that the vast majority of people doesn't really have too much interest in rich RPG and in fact enjoys a theme-park style of game, and even "grinding" (even while complaining, they enjoy it). I couldn't explain otherwise why such games are so massively successful.