eBay feedback

It always amazes and annoys me how many people on eBay fail to leave feedback. When I'm selling, I always positive feedback once payment is received and cleared; what else is there to wait for? But most sellers don't do feedback until after the buyer does feedback, maybe out of some desire to be able to counter nasty feedback with nasty feedback or something. And when I'm buying, once I know the product is what it's supposed to be, I always leave feedback, but most of my buyers never get around to it.

It's irritating, but I wonder what causes it. Are they just too busy or too lazy? If so, that's kind of selfish, since part of the way eBay works is that everyone does their share of making it work. Do they think there's some advantage to them, in which case, they're just being selfish ala the Prisoner's Dilemma, counting on that enough other people will do their share to keep the system working? Are they simply disorganized? Do they just not realize how feedback works, or that they're supposed to do it? In the latter case, is it a special case of not recognizing how these kinds of aggregate systems work, or believing (perhaps from a past bad experience) that it doesn't actually work, or do they think they're not supposed to leave feedback unless something special happens?

Obviously it's some of all of those things for various people and in most cases they probably just haven't thought about it at all. But it seems to happen to me a lot, maybe half the time. It irks me. I always send what I promised to send!