While forex is set by the network, some cards charge a cross border assessment (CBA) fee on international purchases.

Though admittedly I have 3 cards with no CBA and only that charges a 3.8% CBA... but I did prioritize for cards better for international travel. Most of my friends have cards that do charge a CBA

MC provide a currency conversion rate, yes, but a lot of card issuers also add a foreign transaction fee of some kind on top of that, sometimes both a percentage and a fixed charge.