They don't really say why they built their own rather than using one of the open source solutions like https://github.com/bionic-gpt/bionic-gpt
In the first paragraph:
> No existing tool could handle the data security requirements and specific workflows Stripe needed
Size and scale, I would think. An out of the box solution probably doesn't quite have the same capabilities as something they can (and now have to) manage in it's entirety.
Stripe probably WANTS to be opinionated about how their company works with the tools.