They don't.
It's a 'loss leader' to draw customers into using the card later on, when interest will be charged.
Even if a card doesn't offer a '0%' introductory deal, lenders can still end up making no money if the card holder pays back the full amount at the end of every month. It's for that reason that someone who always pays back the full amount, rather than paying over time, is likely to have a lower credit rating with many lenders. (i.e. other lenders will be less likely to offer a new card to someone who always pays in full each month,making them no profit, than to someone who always pays on time but not at the full amount).