The Costco (or any other retailer co-branded card) vs. Apple thing is interesting

People run all kinds of purchases through Costco. Weekly grocery runs. Running their business, etc. People generally aren't shopping at Apple on a weekly or even monthly basis. The 3% cash back on Apple purchases just isn't as lucrative as rewards on spending a lot at Costco/Walmart/Target, and don't result in saving as much on credit card fees that the retailers get on their co-branded cards

I suspect there's a fair amount of overlap between the two populations, which is why I thought the comparison would be interesting. Both tend to attract affluent households.