A Brazilian CA trusted only by Microsoft has issued a certificate for google.com

This is a bad look. I expected the result would be Chrome and Firefox dropping trust for this CA, but they already don't trust this CA. Arguably, Microsoft/Windows trusting a CA that the other big players choose not to trust is an even worse look for Microsoft.

What is even the point of a web CA that isn't trusted by all of the major players? Is there one?

These are generally government CAs, so, typically the situation is Microsoft sold the government Windows, and as part of that deal (at least tacitly) agreed to the CA being trusted, and so every system that's trusting these certificates is a Windows PC anyway, running Edge because the whole point was the government will only use Windows and pays Microsoft $$$.

Why bake it into everybody else's Windows? If you make say a Brazil Government-only Windows which trusts this CA instead, I guarantee somebody crucial in Brazil will buy a 3rd party Windows laptop independently and it doesn't work with this CA's certificates and that ends up as Microsoft's problem to fix, so, easier to just have every Windows device trust the CA.

They'll have an assurance from the CA that it won't do this sort of crap, and that's enough, plausible deniability. Microsoft will say they take this "very seriously" and do nothing and it'll blow over. After all this stuff happened before and it'll happen again, and Windows will remain very popular.

what's the state's interest in having their CA built into windows?

States are themselves extraordinarily large IT enterprises, they generally want control of traffic and its transparency or protection, and they are large enough to get arrangements for that, though usually not this particular arrangement.

Large enterprises in the US generally have the same capability, but not loaded into operating systems by default (that is: Walmart's ability to do this on its own network in no way impacts you, who have never worked on that network).