Because before banks actually had consumer ethics and didn't like taking poor peoples houses from them, but then they found out that there is a lot of money to be made buying and selling debt.
cuz then they know you can pay it back. isn't that obvious? or not, maybe, i don't know.
for the same reasons why we need money to pay for college to get a job to make the money we need to pay for college. You could get a job without college, but then you need the experience of already doing that specific job just to get employed for that job in most cases. Our system doesnt make sense, but at least its possible to get away with murder :)