Their eyes are bigger than their stomachs.
I'm sure the vast majority of purchases are made with the intention of playing through it eventually, so they take advantage of the deal while it last despite already having a backlog of games. That's why, for instance, the gamers here (Not the collectors) have such backlogs. I've got hundreds of games yet to be played, but each and everyone of them was purchased with the intention of eventually giving it at least some attention. I'm not building a collection and just purchase games that I actually do want to play, but you can still build up a large amount of unplayed games that way.
I don't think there is much concern about Steam going away anytime soon. Valve is a healthy company and Steam has been a successful distribution program that is just going to become more common and accepted down the road. So some unplayed games sitting on your Steam list isn't a huge concern
Plus, for many people, including myself, PC gaming is somehow more disposable. I don't keep a PC from 1990, 1995, 2000, and such around to play games from those eras. And trying to do something like play a MSDOS game from 1995 isn't really convenient with a modern PC. PC gaming, with few exceptions (Half-Life, for instance, is a game I'll revisit every few years), is something I experience at the moment with no intentions of revisiting it years down the road. So not actually owning a game is easier to put up with than on a handheld or console (Although I still don't like it).