I hit me not long ago. Trying to do a trade chat flex raid. So we go in and have 2 wipes and people leave. Another raid, 1 wipe, some people leave, some being guildless as well (hint).
Then I got it.
Some people are just not suited for anything that requires a bit of patience and dedication. They can't be part of a pve guild. They are just unable. It's not because they are bad players, though some are that too, it's just that they no fucking patience at all. They also lack basic social skills and find that being unable to call people noobs or retards is a big problem for them since they get kicked from guilds for doing that.
So that's it, that is why LFR is so successful. Because some people, quite a lot of them, are only worth for LFR and trade chat pugs and that's the only way they can progress.