They aren't a payment processor, they are a payment provider. You are correct that most banks have deals with Apple and thus cards are available, but Paypal, Venmo, Cashapp, Google Pay, etc. can't be used as a default payment provider for purchases on the iPhone.
Apple reportedly makes about 0.15% of each purchase through Apple Pay[1].
Of course they can’t, because they’re not credit/debit cards. What’s the PoS going to do with a PayPal NFC? Nothing. It’s like complaining they don’t support bitcoin - neither do the stores!
There are lots of payment services that are not credit/debit cards and are widely used (swish in Sweden is another one). All these cannot use NFC on IOS.
Apple reportedly makes about 0.15% of each purchase through Apple Pay[1].
[1] http://www.macrumors.com/2014/09/12/more-apple-pay-details/