Having paypal available within stripe integration would simplify a lot of things (not sure if it would really help save cost), I think Stripe has already made this possible for Europe. It is probably coming soon for the US as well
> PayPal enables XS2A use cases for TPPs through PayPal’s REST stack. Through PayPal's reliable and proven APIs, TPPs can access the same PayPal systems that power all of PayPal's merchant and consumer experiences.
Stripe isn't a TPP and even if it were to use an external TPP (Tink, Truelayer etc) to allow users to initiate a transaction from a user's PayPal account (similar to A2A transfer), it would be significantly "less integrated" of an experience. There are many problems with the UX around Open Banking A2A payments and it would harm conversion. Because of these reasons, I think it's a bespoke integration/deal between Stripe and Paypal
That competitive pressure isn't nearly as strong in the US, and Stripe would be loath to be sending money to their direct competitor if they don't have to.