It looks like most 5% cash back cards have a hard limit ($1500/quarter seems to be common limit) to the amount you can get back and offer an "unlimited" cash back at a rate at or lower than 2%.

https://www.magnifymoney.com/blog/best-of/10-best-5-cash-bac...

This way they'll make money after the limit takes effect. My assumption is that they are hoping the consumer will forget there's a hard limit to the amount they can save and always go with their "5% cash back card" when making purchases.