Visa has a market share of ~48%, MC ~36%, and the rest if split up between smaller cards. There is a "peaceful equilibrium" on the market (some would call it a cartel, but not me! No sir!). Why would they want to merge? It is virtually impossible at this time for any new player to enter the market (mostly due to regulatory requirements), so they can maximize their profits any way they see it fit.
In case there would be something threatening their dominant position, that would change the equation, of course...