Because some ppl take better care of their stuff than others... I dunno... I don't touch any of my videogame stuff without having clean hands (ie washing them) first...
I also clean my controllers so that when my naive friends play and their hands are all sticky or gross or they just sweat a lot I don't have to worry about grabbing a dirty controller later on... plus its nice using a clean controller than an oily one...
That and I generally take care of my stuff better than most other people, I don't leave stuff out in the sunlight, put my games back when I'm done playing (don't leave it in the ps2, or take it out and leave the disc laying somewhere) almost always put my games back with the cases standing up (other than my boxed stuff like Growlanser, Kof MI, etc), I don't smoke, clean regularly, have my games organized, etc etc...
I guess its peace of mind. That, and I don't mind waiting for the price to go down. I won't buy a game used unless thats pratically the only option (facing paying much much more for a new sealed copy).