Almost every American, it seems, gets a new credit card offer in the mail almost every week.
Credit cards are highly profitable, but only if the customer stays around for a while.
It is a tough business. No matter how low you get, some rival can always undercut you. You teach your customers to think of your card as a commodity, and to shop only for price. After the six months is up, they will be receptive to the next lowball offer that comes along. You will lose the customer, and the money you spent to acquire her.