It's a simple marketplace and they control a ton of the demand side who trust them to find the best price and providers that work. The value is huge to users - one place, multiple models, providers bid for the traffic and the user doesn't have to think about anything. The value they hold hostage against providers is gargantuan - nice model there it'd be a real shame if none of our users used it.

I don't see how stripe adds any value here (and I've had such terrible experience with stripe automatically breaking my stuff I am worried I can't trust openrouter now) but I can see stripe wanting to be in the middle of any two people giving each other money on the internet and this is squarely (lol) that.