Because when you offer up a Randy Moss, you might get a good player to replace him PLUS throw-ins that help fill needs. Moss, while a talented player, is also a huge distraction. Moss is also not capable of carrying the team to the SuperBowl.
When you add up the +/-, Moss is definitely on the plus side, but that's simply not enough. If you can make a trade where you get even more upside than Moss when evaluating the trade as a whole, it quickly becomes worth making the deal.
While generally, the team that gets the best player in the deal "wins" the deal, when you're trying to move up a level, sometimes, it's depth of talent that you need to build.
Make no mistake - giving Moss away for little in return is a bad, bad, bad idea. But someone will value the guy. That's Minnesota's cue to ROB THEM.