Stripe could choose to support FedNow instant payments and push these funds for pennies (up to $100k at a time), assuming the receiving institution hosting the deposit account(s) is set to receive on those rails. They would arrive within 20 seconds, per FedNow’s SLA.
Only costs ~$25/month to plug into these rails, plus a few cents per transaction.
Congress directed the Fed to create FedNow so the largest commercial banks couldn’t gate smaller regional banks access to real time payments via the private consortium The Clearinghouse (owned by the banks you list). FedNow directly competes with this private network. Think what IRS’ Direct File is to TurboTax, reducing unnecessary economic drag at scale.
Those listed(and many others) support RTP, which came first and accomplished the same thing. They probably see little incentive in adding FedNow any time soon.
But then they wouldn't be able to charge more fees