What are they providing? Stripe is supposed to be simple, I'm not sure how it could be much simpler without losing out on fidelity.

I just integrated it into my web app and although it took longer than I would have liked, they all seem like necessary components.

Using a SIAAS would 1) expose a larger surface area for trouble around sensitive matters 2) cause them to deal with things that I didn't explicitly specify granted that their abstraction will be leaky