In Warcraft there are three types of people really, the casual, the player, and the raider, there are of course subdivisions with very casual players, players that raid, raiders that do it once a week or month or raiders that spend 12hrs a day raiding. Its been said more than once about truly hardcore guilds as to how can they waste so much time raiding, yet overall they clear the content in fewer hours than "casual" raiders, its just massively condensed rather than spread over many months. The question is thus what makes a raider raid.The answer to this is complex, and could probably be someone's thesis, and the most likely answer is, because we like raiding. This becomes very apparent over summer, guilds slow down, raids get cancelled and guilds fall apart. The truly hardcore guilds in many ways have it the worst, maintaining only a small population of spare raiders (otherwise the capability to get slots in a raid is too low with high attendance per head) means that a few people taking a break or holiday can stop a raid. This problem though is not limited to them, Guilds with larger populations, but less attendance per head have the same problem, if their core raiders stop or take a break or those people who can be counted on to come "If I have to to make it go" suddenly the pressure to raid drops, and with it the 24 people that didn't get to raid feel disappointed, and rather let down. Raiding isn't like a 5 man instance or grinding, its not something you can say "Lets go down Illidan tonight" and find the people to do it, all fully buffed and willing to spend the time and effort to kill him (ok thats a lie, once you have sufficient gear and a skilled core you can randomly do raids that are "farm" level to your core's gear, so for T5/T6 core Gruul and Magtheridon can be easily done this way).
So what is it that is attractive, it's a lot of work, its costly, takes a large amount of time (even just the trash clears normally take 30minutes to an hour), but you get a great feeling, a group of players working together, a huge challenge and a task which requires every member to achieve their goals in order to actually down the bosses. Its like any team sport, its co-ordination, working together and achieving as a group, its conquering the game* and knowing that you tried your hardest, got the best gear and with those team mates and the people sitting on the sidelines finally triumphed.
So why do guilds fall apart and why is it so divisive, I think I am an example, my previous guild hit the summer stage, people going away, not raiding, coupled with a couple of people basically leaving or giving up raiding. We were already pushed to make 25 man raids normally, with a raiding core of 10-15 people so we had 15-10 casual or players in the raid who were along because it seemed like fun or to help the guild. In short we had a small core that could likely down Illidan in a full raid, but we had others for whom killing a progression boss and seeing the loot drop wasn't a major deal. Suffice to say summer and holidays hit us hard, and essentially 2 months of cancelled or ineffectual raiding. At this stage people started to jump ship, mostly raiders going to bigger guilds, normally saying they "wanted to see the content", but privately most of them liked being in the guild they were in, they just wanted to raid even if we never saw Illidan. Eventually I jumped ship as well, I didn't have a heading I just wanted to raid, I spent a week or so unguilded (by choice, I got recruiting /w's as a tank from about the 8th-27th guilds on our server within an hour of leaving), and I was bored. Instances with PUGs can be fun, but it doesn't replace the friendship and experience of raiding with friends, doing the farmable raids (Gruul and Magtheridon, parts of SSC and TK) with people I know when they had time didn't really fill me with joy, sure they are fun but its similar to going back to normal instances, its not a challenge because you can only lose if people mess up.
That sounds silly of course, most raid bosses are killable if no one messes up, and most wipes are caused by someone messing up. So what difference does it make to do it on Illidan than Gruul, simply its a challenge, the degree of complexity doesn't really change, it was hard to kill Mr The Dragon Slayer (back before we were on first name terms) when we were in Karazhan and Heroic quality gear, yet now though his complexity remains the same it doesn't matter if the tank gets crushed or has a bad avoidance run because his health pool will cover it, and in truth is harder to not avoid the damage being taken than it is to avoid it, the fight stresses the tank in the other way, finding the way to build threat.
There isn't really an answer to this, once you have the gear level to out-perform content its challenge is reduced to your group, Heroics were once hard, Karazhan's Curator an uncrossable boundary to upper Karazhan and the concept of achieving four Zul'aman timers within 45 minutes a feat that seemed impossible. Yet these tasks are all now doable, the group hasn't really changed, the skills and techniques demonstrated don't change, the whole of the Burning Crusade can be summed up as "Stay out of the Fire", with fire replaced by an appropriate element. What changes is our gear level, and thus our expectations, as you gain the ability to down bosses that once seemed impossible, to move through attunements and become a raider it becomes harder to take a step back and look at other content as an achievement in the same way, its still fun but it doesn't get your blood pumping. Guilds tend to fall apart because of this, they get their players to the stage of seeing the next step of walking forwards and being on the path. When the path hits a stop or a fork, a small river runs across it and you need to ford it its often easier to go to a different path that hasn't had this problem yet. You lose the sense of friendship, and the bonds that drew you to these people, but since a lot of what you did together was raiding, and you aren't seeing them all anyway because you aren't raiding the sense of separation increases.
There is never a solution to this, no single guild can offer enough raiders slots to raid without having excessive rotations such that when a lean time hits it survives, guild alliances are often risky, if the other guild is better you might lose players to them or them to you, and what happens about dkp and such like, its a very complex situation to get into just to cover a lean winter. Perhaps the solution is a Raiding Alliance, essentially abstracting the raiding part of a guild to a third party group, where a huge number of players can be present with the alliance running 2,3,10 or 100 raids a night each targeted at the level of performance and gear that the players have, I believe there are some running upwards of 400 raiders with 90% getting to raid on the nights they want to, and doing it successfully. It takes the DKP and guild alliance problems away because the alliance handles that aspect, experienced raid leaders run groups within it and have backups to ensure that a raid isn't cancelled due to lacking a leader. Maybe its not a solution, but I think a lot of guilds, servers and people could really learn from this kind of system, something to let your raiders raid and maintain their social aspect within the guild, many raiders don't want to lose that, but they also don't want to stop raiding, and while you can talk to friends outside of your guild, you can't normally raid outwith it successfully.
* This depends on your perspective, in some ways downing Illidan finished 2.3, killing Kil'jaden finishes TBC as a whole, but many people never consider this a goal, so really the game ends when you feel truly satisfied with where you are, but from a raiding perspective we must consider the end bosses the final state.