Most of your cash back money actually comes from fees that merchants pay. In the US and especially for credit cards with cash back those fees can be quite high, and unfortunately it's illegal for merchants to discriminate against credit cards with high fees.
> it's illegal for merchants to discriminate against credit cards with high fees
Do you have a source for this? Often times merchants don't accept American Express because it charges higher fees [1].
[1] https://www.bankrate.com/credit-cards/business/why-american-...