To get 5K in rebates assuming 1% cash back you would have to charge 500,000.
But your point is still valid. If you go to IP and spend a grand and pay cash you spent a grand. If I go there and hit it with a 1% card I spent 990 bucks. The only way cash is a better deal is if there is a cash discount available or I don't pay my grand off at the end of the month.
Hell, I paid for 3K of my wifes car with a credit card, that was the max the dealer would allow. 30 bucks is a tank of gas for her car. Free. If we had just paid cash for the whole car we would not have gotten that.
It isn't about how much the credit card company makes, it is about how much you pay, net after cash back.