I'd like to explore why it's so much. I get that same feeling, but is it only the fact that you're so free that paralyzes you which choice to make? Or is it that none of the choices seem to provide benefit to you so you don't really feel like doing any of them?
The "why" of it really boils down to target audience. It's the same reason why some people prefer racing games over shooters, adventure games over survival horror, and action rpg's over turn-based.
Without having much experience with online gaming, I'd speculate that the people who are most into MMO's are going to appreciate non-linear games more so than those who are not.
With MMO's, you're really only limited to what you can program and the membership-base. As long as you have a strong following, you can continue to add to the story and gameplay by including more content.
With single player games, this is much more limited, outside of expecting your audience to keep downloading content whenever it's provided. Therefore, most single-based games (at least the rpg's) generally have an end-goal, where the game is finished. That end goal can change based upon actions are taken, but there generally has to be an end somewhere.
As for why non-linear would be too much, it really has to do with time, I think. If the game is only going to be about 5 hours long, I won't be bothered about non-linearity, and I'll probably enjoy it. If it's about 50 hours long, I just don't have the time. Then again, I barely have the time for a 50 hour linear rpg. But being that time is of essence, a more linear game is favorable, because at least I always know where to go and what to do.
Not to mention I grew up with j-rpg's that were story driven, and so I tend to favor these types more.
However, for me, the best ones incorporate a mix of linear and non-linear. For those who want the main story, they can stick to the path, and those who want additional content can deviate from that path by finding sub quests, special monsters, additional story segments, or even sub-plots.