To ensure agents are paid through them and maintain market position https://stripe.com/blog/giving-agents-the-ability-to-pay

See projects.dev by Stripe. My best hunch is - Stripe wants to get in on the infrastructure/rails layers being built for AI. They aren't sure what that is going to be exactly (nobody does). OpenRouter seems like a safe bet.

Stripe controls parts of APIs for money movement across world. This is just a logical extension to that.