It seems to that guilds become more collectivist, or display more of the attributes associated with collectivism, as they get more and more hardcore. Loot Council, higher attendance requirements, higher performance requirements, more shaming for those who don't meet said requirements, more expectations in general.
Guilds at a higher level demand more of their individual members, and thus are more able to act as a single entity. So why would these players chose to subordinate themselves to the group?
I think the key is reciprocity. A player is more willing to sacrifice if she can be assured that everyone else in the group is sacrificing as well. If she is assured that everyone else is as committed to the guild as she is.
As well, this form of collectivism cannot be imposed on people. The right of exit in MMOs is very strong. It's trivial to leave a guild, or even to quit the game entirely. Every collective in an MMO is a collective because the individual members choose to be so.
This is, of course, very different from collectivism in the real world. Very often, the collective is enforced through shame and other rules. And they usually have tons of "free riders" who exploit the collective for their own benefit.
I wonder if that's a way of looking at the extended endgame. Rather than one collective, or a mass of individuals, it's really several smaller collectives, each with slightly different properties.