Story Time; Throwaway because it's very personal;
Growing up with divorced parents and witnessing this first hand is disgusting. My parents split when I was little and my Mother received custody of me. My Dad had done nothing wrong,(wasn't a drunk, abusive,mad man, etc.) to not get custody of me, that's just how the courts decided. So, after all is said and done income is shown and my Mom gets a certain amount per month (I believe it was 1k back in 2001). My needs weren't that much, but just your typical child things (Clothes,meals,shelter). No where near 1k a month. Years go by and as I continue to grow, my Mom brings my Dad back to court. She finds out he had the highest paying year he ever had and decides to sue him for more money per month, claiming she didn't receive enough support. After months of battling, he loses (to my manipulative mother), and ends up having to pay 2.5k a month. This went on for about 6 more years until I was old enough to tell the courts to stop bleeding my Dad for every cent he owned. My mother, (unemployed since the divorce) received 30k a year tax free. My needs were about 5k for a year, max in the end. I never saw a dime of the child support. No college fund, no bank account, had to buy all my own things, just ridiculousness.
The reason I share that is because there is no true explanation for income based support compared to child's needs per month. The only reason there is, is greed and corruption. The courts are skewed to a mothers favor 100% of the time, while the fathers are looked down upon as deadbeats. Because today's society, everyone is a victim and no responsibility is put upon themselves, it's always someone else.
TL;DR: CS is done by income because of the love for money.