Earlier quoted context omitted.
Adoption. You'd have to onboard hundreds/thousands of banks and terminal providers so they accept/give out your card. I excpect the underlying technical stuff isn't that hard compared to getting people and companies to actually use it.
Aren't cards last century technology? I'm paying with my phone anyways. Seller can use phone as well. Why does it need to involve incumbent banks and terminal providers at all? If Valve started something like that the banks would bang on its door relentlessly just to not be left out of the loop. Gaming is the business bigger than movies, music and books combined and Valve is Google of games.
And what in your mind is the thing banks will be begging Steam to be let in on? This reads like payment processing fan fiction.