Credit card issuers charge a percentage of the transaction. MasterCard and VISA add a small flat-fee per transaction, but American Express typically does not. Big Spenders do not only make large purchases, they also make more purchases. It's not really a comparison between 1 X $10,000 vs 10 X $100. It's a comparison between $10,000 per month versus $1,500 per month.
High income individuals are also more valuable for marketing purposes. Issuers sell access to their cardholders to merchants.
Is there any interest in middle to upper-middle class spenders? Or is it only about BIG spenders?
I now understand why AMEX wants to do business with big spenders, but why did they bother with nickel and dime Costco customers.