The same can be said for autos...any player who is an untested prospect (and obviously never makes it). How many 100s or 1000s of signed cards are floating around out there now that not even dedicated team collectors want (unless they are dirt cheap!)?
Unlike autos, which PACK products with no-name prospects who never make it, GU cards seem to stick with the more seasoned players though. So you usually get someone who played a little while or was maybe even an all-star a season or two.