At scale, payment processors are amongst the most difficult things you could do because every two bit crook out there is going to try to scam you somehow.

Can probably build it in a weekend.

The thing is they have "tooling" to help create their own customers:

https://stripe.com/atlas

They also have a tax product, and a few other things that are in the orbit around payment processing.

Their product offerings are a bit more than just the "dumb pipe" portion of the transaction.