Each $1 of stable coin is supposed to be backed by $1 of dollar or short term equivalent. So the issuer is making money by collecting interest on it.

3-4% of billions (USDC alone is $80 billion) would itself be billions of dollars of annual interest. Easily covering the operating cost of these companies.

However, they don’t keep it all. Nobody is going to let you hold their cash in size without getting a slice of the interest. All the big players (like an exchange holding USDC of its patrons) cut deals with the stable coin issuers for a revenue split of that interest.