It would take a lot of effort for Mastercard/Visa to stop physical retailers from selling Steam gift cards. Beyond gift cards, there's also systems such as PaySafeCard, which lets you pay with cash at a physical store and spend it online at any merchant who accepts it using a code.

And for crypto they can just accept Monero. Steam accepted Bitcoin years ago, but stopped due to high fees and network congestion. Monero fixes that + makes it private like cash, and has been the de facto cryptocurrency for years now.[1]

[1] Random example https://xcancel.com/NanoGPTcom/status/1951300996329537625#m