Depends how much you bounce around between games, really. A lot of gamers do not focus on one game at a time and rarely finish them, so the library can stack up quickly, when you always crave something fresh and new. Especially with games as big as they are these days. What's the average size? About 40 gigs for the big boy games, give or take? Doesn't take much for the scatterbrain gamer to fill that space up. Then there are the ridiculously large games, like Halo 5, or RDR2, that eats a tenth of the 1TB HD alone.
It's not a problem I have, but I can certainly see others filling up 1TB rather quickly.