I think you may be missing the fact that a lawyer, doctor, accountant etc has ~40 years after they graduate to earn $big, an NRL player has maybe ~10 years after they "graduate". So they have to maximise their earnings for that short period of time, because after that the vast majority of them have nothing much to earn an income from. A %couple end up in the media somewhere, a %couple end up coaching, in administration etc and the rest may have used their "education" payments to actually set themselves up for a career after football. The 90% end up driving trucks, cleaning up garbage, laying bricks or digging ditches. I don't begrudge a professional sportsperson earning while they can, because it doesn't last that long.
Go Dogs