Because it's there.
I think raiding guilds generally are having a mid-life crisis. They want to recruit good people but people that in the past might have been available for recruiting are happy enough with LFR. Guilds haven't figured it out yet and doing something like a farm system is too much work, especially for a smaller raid group. So casuals and lesser-skilled people are fine with things the way they are now and happy enough to avoid being scheduled to death and the drama that goes along with it.
The raiding community has always given off a public vibe of being a community in which membership is earned. A lot of people don't feel the need to play that game any longer.
For those with the skills and interest, 10-man guilds are far easier to find.