Well for me it's fits of unparalled rage combined with a lack of suppo...oh right, you mean piracy. I thought you were talking about that unfortunate incident with the fire axe...
Seriously though, I think when it comes to hacking, it can be for a variety of reasons. I'm sure most of it is just to save money, but on the other hand, rom hacks are a good way for up and comers to practise their programming skills. Hell, there are some rom hacks out there that are so vastly superior to the original file that I'd rather give the hackers my money. The many Pokemon hacks that remove the need for trading, hacks that fix game-breaking errors, or my personal favourite at the moment, a hack of Sonic the Hedgehog 2 I found that totally restores the missing stages. Granted, the Death Egg zone is a re-skinned Chemical Plant Zone (yeah, that's right, we noticed. Nice try but no) but the restoration of levels like the Wood Zone, Dust Hill, Genocide City (both acts are the same there but better than nothing), etc, finally allows me to put an end to the mystery of the 10th song in the sound select menu by not only knowing about Hidden Palace Zone (many thanks, Pop Fiction) but having actually finished it. A childhood dream realised, and all thanks to hackers. I'd like to think that some of them do it to allow their fellow gamers those wonderful moments.