Why? Because if I decided to play an old multiplayer game, I know that there's a chance of it still having a playerbase.
For instance, you do not hear about anyone playing Left 4 Dead 2 on Xbox 360 anymore, but you'll usually hear about it being played on the PC.
Diablo 2, a game which came out in 2000 is still being played despite Diablo 3 being released.
No hooking up old systems to play anything, nothing.
Also, I am not hating on consoles, but really, it's much more convenient to play on a PC and with Steam it can end up being cheaper. Right now on Steam you can buy both Deus Ex games and all it's DLC for $14.99. How cheap can a console game get non-used? $19.99 at best.
Also, and I am not promoting this, if you're really desperate to get a game, you can always just pirate (only if desperate).
Think about it.
Please comment if you have something to say. :]