Too many other games to play to spend time replaying ones I've already finished. As far as I'm concerned New Game+ is pretty much the most worthless feature a game can have, with the sole exception to that being Demon's Souls. And nothing irritates me more than an additional difficulty or game mode that is only unlocked after beating it once.
Well, I certainly can not, nor will I speak for everyone else, but I replay games all the time. As a matter of fact, I've spent more of my time playing games from before this gen than I've played games from this gen.
As a matter of fact, I just beat Megaman Legends for the 10th time (estimate), and I am now playing MML2 for the 10th time (also an estimate). I then plan to play and beat CastleVania Symphony of the Night again, and then Chrono Tigger for the 3rd time, and then Chrono Cross.
Chrono Cross is the only game out of all those that I have not beat.
The reason I am playing this has nothing to do with how many gmes I have, I have tons of games... About 100 games, actually. I am replaying these games just because I find them to be great games in my opinion.
There are very few games I can replay over and over again. I just get bored of most after the first playthrough that I just move on to something else. I have around 5 games in my backlog I still need to play anyways. Like that other guy said, it was mainly a lack of funds as a child.
I've noticed niche gamers really like to replays games and devote time to one game. If you're only a FPS player, you'll probably devote alot of time to CoD multiplayer, if you're an RPG player only, you'll probably do 12 play throughs of Mass Effect and Skyrim, if you like JRPG's, you'll probably grind the **** out of the post games and do it again.
As a jack of all genres, I really can only get a playthrough out of most games, and don't really even have time to devote to playing multiplayer.
Because most of today's games don't have fun gameplay, are linear and story-based(not good ones either), and don't give enough incentive to play through it more than one or two times(no new game+, special items, unlock-able cheat codes, meaningful/limited character development, etc.)
I don't have so few games as I did when I was a kid however I also have a wife and a daughter now that I have to manage my time with and there's just so much sitting on my backlog I don't have time to replay anything especially when some games are huge time sinks.