Because we want to do the raid, but we want to do it the way we want. The LFR way, and we're happy for others to do it the pug way, the normal way, the heroic way or the mythioc way.
But we either don't want/can't join a regular raid and/or we don't want to deal with the PuG scene. For every one person here that posts "it's so easy, I pugged blah out of blah!" I'm sure there are many more that faced a night of "Declines" or "No"s....and for all we know, that person faces the same next week when they want to run again.
LFR Can't say "No" it can only say "Come back when you get a bit more gear". Worse yet, even if you do get kicked by the group...30 minutes later, you can requeue into a new group that won't know you've been kicked (Plus if somebody gets kicked often enough, there seems to be an immunity system. Ever wondered why some people have 3 or 4 hours timers before they can be kciked (I've been told that's) why)
Better yet, when I want to :FR, I can queue, see the average time and decide if I have enough time that session to both get in and have time to finish the run.
If I PuG...who knows how long it'll take to fill a new group, or get accepted to somebody elses.
Blizzard gave us tools to avoid the nightmare that is pugging for those of us for who it is a nightmare...so yes, we're quite bitter they're trying to take that away from us.