There are lots of other reason as well I'm sure... but there are three big factors I can think of...
1. Challenge - either the game is too hard or too easy, the hack itself would be a challenge worth overtaking, or the difficulty of hacking the game is so rediculously easy that it's hard not to.
2. Money - the game costs too much (simply costs too much, or doesn't give enough for the price and the hacker wants to make the game do more to feel like it was money well spent), the person doesn't have enough money, or the person doesn't want to spend the money they have on the game.
3. Bragging rights - they want to say they've done something in the game that they can't do as easily without hacking, or they want to say that they've hacked the game.