It's partly because I have been burned in the past wherein I'd friend people back and then since we never really talked in the first place, while I got into reading them, they disappeared, stopped commenting and it felt like an unrequited relationship. My f-list is actually pretty private now as a result. People are free to lurk, but friending me doesn't mean that I'll get around to friending back any time soon.
And you don't have to friend me - you can track posts, or do RSS feeds. If you feel hurt by my stance, you do have alternative options to friending.
I also think friending everyone whose been on the flist a while and filtering them out when I read the flist is a bigger insult than not friending them back. I want to be able to say that I read my flist and try to connect with the people. I connect mainly through fandom posts. I'm not going to friend/keep someone on if I'm not willing to give them at least that much attention.
I'm also someone who is normally a GIANT lurker. Participation in fandom things doesn't actually come normally. Three times in the past, I've tried to do a couple of big, awesome things - but in the end I flaked out, avoided commitments and then discovered it was too late. And yeah, I think of those things and I feel like the biggest asshat ever.
LJ is all about fandom for me - its something that I've needed but never had - ever since I was in middle school really. It's an outlet and even a bit of escapism from RL to just enjoy and share the love. I try to ignore wank apart from watching it while eating popcorn, 'cause that's not why I'm here. Truthfully, if LJ does become more drama than its worth, I know I'll probably log off one day and never come back. That's just me.
For now though, LJ is still a pretty awesome place :)