From what I can find, this is the general idea: In older times, children of poor parents took the route of learning to become civil servants or imperial scholars while the rich were able to pay for children to receive martial training and become high-ranking soldiers. To try and put it into a saying: The rich fight while the poor write.
To become a scholar, you just have to read books and study, but to become a warrior, you have to spend money to buy medicine for your body, armor and weapons to arm yourself, and food for your increasing appetite.