Because I can, seriously.
When you can get your games for free, why would you buy them? I only got MKWii recently because of the crappy 3x speed, and I can't get the backup-loader(s) to work. And the Wheel!
But I'm not going to say because I can't afford them since I sort of can't, but I just pirate because its there, like if I see an iPod on the street, I'd take it, if someone offered a dollar, I'd take it.
The only games worth buying are maybe the ones with add-ons/extra items needed such as: Guitar Hero(all of them, if the World Tour guitar is compatible[probably is] with GH3 I'll get it since I like some of the music. I'll let my cousin get RB2 since he has 1, and I want to play Chop Suey!
Same with On Tour) and Mario Kart Wii, since it's really good.
I don't usually pirate for consoles though, if I get a 360 or something, I'll buy some games and Xbox Live! so I can play online for real.(Nintendo Wi-fi sucks hard, PSN is okay...)