Nope, you nailed it. There is nothing in payments that requires more dev work than could fit in an Excel function. Patrick admitted as much with this screenshot of the complete Stripe codebase earlier this week: https://twitter.com/patrickc/status/381779690567913472

That said, the pingpong tables aren't going to play themselves...

Payments is an extremely hard problem. Try picking up a NACHA rulebook. Makes a great doorstop, it's the size of an old-school phone book.

Don't know what NACHA is? Consider the possibility that you don't actually understand the problem space, then.

You are missing an understanding of the complexities of the credit card payments industry.

I suspect it's to do with the 100% uptime they should be aiming for. They might have some other interesting stuff they're working on as well.