Valve could have said "fuck off, thats non of your business" to VISA. Visa MASSIVELY profits from Steam
Except then Visa pulls support from Steam and possibly hundreds of thousands of devs are impacted as their games are unable to sell. It's a massively shitty situation but Steam and Itchio really are trying to hold on however they can. Everyone profits from business, but the devs are the one with the thinnest pockets, and the ones who are gonna get hit hardest if that business is disrupted.
Would refusing Visa work? Sure, probably even sooner than later, but Valve and Itchio has no right to make that decision for all of the devs using their platform. We already saw what a few days of disruption did to everyone, could devs hold out for weeks, maybe months? It's effectively a union strike, which requires the agreement of everyone involved and willingness/ability to hold out.
There are definitely ways it could be done, but it is much more complicated than just Steam deciding to do it. Really just comes back to the question of, How is Visa/Mastercard so stupid as to get convinced to do this in the first place?