If only because I never finished them in the first place... there are so many games that I've played halfway through and had to restart its ridiculous. More often than not its due to human error: I got about 3/4s through Final Fantasy VII back when it came out but could never finish it because I ruined the disc. I was playing FFVIII on my computer and lost my save file about halfway through when the computer crashed. I didn't have a memory card in the PS2 when I was playing Bully so I got about 3/4s through until my mom accidentally turned the system off. Shenmue, Kingdom Hearts 1 and 2, LoZ Windwaker, EDF 2017, Chrono Trigger (the DS one) to name a few were all victims of sales, trade-ins, etc. when I would make it about halfway through but figure on a whim I would never want to play them again.... but of course I keep coming back, restarting from scratch.
There are a few games that I play multiple time simply because I enjoy them so much... I've played Arkham Asylum twice now (and I feel compelled to play a third time), I've played San Andreas, GTAIV and the two Liberty City episodes twice, both Modern Warfare games I've played twice, and I've probably put enough hours into Oblivion to have played it about 10 times now. Oh and I've probably played King of the Monsters 2 about 30 times since I was a kid, but that game is only like 15 minutes long anyways. Generally speaking though, there's more games I've played once and tossed aside than games I play multiple times, especially with the lesser emphasis on long campaigns these days.