Ooooor... because they want points? For example I have a capital one venture card with 480,000 points which I plan to use for a vacation to Hawaii next summer. I have not paid a single penny in bank fees since I pay my bill in full every month, and I had the $35 annual fee waived for life as part of a promotion when I opened the card. So I get a free vacation and I have not paid the bank a single penny. Can't do that with a debit card.
ahh points, spending more in order to reach bonuses