Stripe is a payment processor. It has different payment methods it supports. Which are MasterCard and Visa. But it does also support(if you can use it and the merchant accepts it): Giro card, other card options, Revolut Pay and other options. Those avoid the MasterCard, Visa ecosystem.

Regardless, not everyone wants to use or uses Stripe. And it's a bad thing for everyone to use Stripe just because of their interoperability because that then gives stripe too much power.

Plus, for a company like Valve, the fees of Stripe would severely affect their revenue.