However, sites could accept IBAN transfers, Diner’s Club, Bank drafts, Amex, Discover, JCB, Apple Pay (Apple Pay Cash)... People could even use Transferwise.
The are a lot of ways to not have to accept MC or Visa.
Even six years ago when I was shopping around to buy some Bitcoins it was already impossible to use Transferwise and most banks had blocked transfers to known cryptocurrency exchanges. When the financial industry wants something ostracised they are quick to move in the same direction.
Your observation doesn’t change that OP hasn’t seen it.
Amex is a small card processor. According to the Nilson report [0], Visa and MasterCard made up 80% of global transaction dollar amount in 2016. Amex, Discover and JCB combined made up 5%.
You may know of Amex but they are a very small card, especially outside of the US.