Probably because it's a hell of a lot easier than trying to figure out how to manage payment processing without those processors. Visa and Mastercard are extremely large, and by-and-large the only way to pay online in the US. Add in Paypal and Stripe's limitations (which are also notoriously shitty) and you don't really have many options left, so it's really not worth it. I know the EU has better options, but Steam isn't based there and I wouldn't be surprised if they didn't want to find a way to jump through those hoops.