At that moment in my life, the risk on lending me money was high and therefore the product I could access was limited. There was little incentive for the bank to risk lending me money, so I needed to purchase my way into rebuilding my credit. As the risk in lending me money decreased, banks became incentivized to offer me lines of credit so the cash backs kicked in.
Rebuilding my credit has allowed me to access lower rates on my mortgage , investments loans and other financial tools that have greatly contributed to my quality of life. I would consider that to be the reward.
It's frankly a brilliant scam, and the fact that people don't think it's a scam is its most brilliant aspect.