I think it's a mix of a lot of the things mentioned. Owner's often tend to overvalue their own players. They also hold onto the draft pick they spent for him long after it loses it's relevance. Moss may have cost you a top 10 pick 6 weeks ago, but he's obviously not worth that anymore. I also think many are afraid of making a bad deal, and won't pull the trigger unless (as mentioned already) it's a slam dunk in their favor. If you have a solid team already, there's also the added impact of them not wanting to make your team better, even if it improves theirs as well.
I'm in a 12 team league where I'm pretty stacked at RB & WR:
LJ, Jacobs, R Bush, Rudi
TO, Cal J, Bowe, R White, K Walter, Branch
I also have only Rodgers & Garrard at QB, so the shoulder issue has me concerned.
There's another owner who wound up with Romo (who he's started every week) and Cutler at QB, but his RBs & WRs are:
C Johnson, Stewart, M Bush & worse
R Moss, V Jackson & waiver wire scraps
I offered him a few RB + WR for Cutler deals, all of which give him 2 guys he would be starting almost every week, instead of Cutler wasting away on his bench. The guy is 3-1, mostly by good scheduling luck at this point, but if he improved his lineup a bit he'd be able to contend every week w/o luck. He's turned every deal down and says he wouldn't give up Cutler for any deal unless it included TO.
I've tried to explain to him that getting a top 5 WR for Cutler (or any QB other than 2007 Brady, for that matter) is unrealistic, but he won't budge. I've tried to tell him that there's a reason I drafted TO when even #2 rated QB Romo was still on the board, and would do the same if we redrafted today, and he obviously understands the relative value since he took Moss in the first round himself, but he still won't budge. I don't get it...
I don't think I've ever seen a more obvious scenario where 2 teams could both benefit greatly by making a deal, but I doubt it's going to happen.