Browser vendors have an incentive to maintain trust, it's their stock in trade. If they screw up, it directly affects their bottom line because people will move elsewhere. Their goals align closely with my own.
A foreign government (or even my own) has somewhere between zero and weak alignment with my goals. There's little reason they would want to choose good certificate authorities and strong reasons why they would choose ones their intelligence agencies can backdoor. Why should I add them to the chain of trust?
Their stock in trade is being easier to use and more feature-rich than the competition. Trust doesn't factor into it, in the sense that trust in a browser does not come from track record: It's all marketing, all the way down. Thus, if they screw up it does not matter. This point is easy to see because they frequently do screw up and it hasn't had any impact so far. There's no reason to assume it will have one some day.
Browser vendors monetize your data, that is their main trade. Having lots of users doesn't matter if you don't control them in some way, such as decide who gets to be trusted as a CA. The browsers are supporting the current tech hegemony for CA's, for some reason Americans are much more "trustworthy" than people from anywhere else, do you really believe that? Why should the people in Europe be forced to trust shady American companies like Google or Microsoft who are or likely will get compromised by the American state? It makes sense for them to regulate this so that they can have European options.
Why is that?