Sometimes it migth not even be greed, but necessity - and maybe not for mortgage volumes.
I mean, duh, look at me. If I wasn't so lazy in EVe, and also and possessing a sense of morality (and having dealign with it so long as a GM on several game servers fighting it) RMT would put food on my table and clothes on my back that disability money doesn't. Not everyone breaks a TOS just for buying yachts.
Not to say that some don't buy yachts though. but if youre suddenly in a RL tight spot one day, take the chance and get away with it, the ease of success would erode any moral qualms as time goes on.