Only because there’s not been any significant interest to change that. There are alternative already, American Express (which only issues cards for its own bank currently, but do have market share outside the USA), and Discover in the States. They both operate both sides of the system (issuing cards/credits and managing the underlying transaction network) whereas Visa and Mastercard only operate the network.

Visa and MasterCard are by far the largest, but there’s no reason that alternatives can’t succeed (even though it is unlikely).