And legal precedence wise, laws have been requiring things of websites for a long time, but I can't think of any legislation that has required certain behavior by browser applications themselves. Since browsers, in theory, just implement open web standards, requiring something of them is effectively legislating a forced change into web standards. That's a spooky thought.
But let's be real... having individual websites ask for cookie and 3rd party request consent makes opt-out massively less popular than if this was built into the browser. If the "disable ad-targeting cookies" button wasn't hidden behind obtuse UI bespoke to each site, far far more people would opt out of them. I suspect that's a big reason why we got to this arrangement.
The browser has no way of knowing what kind of tracking is going to happen on the server it's connecting to or what jurisdiction that data would fall under.
If trackers would provide metadata indicating what kind of tracking each resource is going to do, then browsers could handle it, but trackers would never do that without being forced because they want to make opting out of tracking as painful as possible.
Then the consent question would actually be presented in a balanced way. And the preference could be stored even if you wipe cookies. At moment if you distrust cookies, your preferences expressing your distrust of cookies is constantly being reset forcing you be re-prompted again and again.
(It might be harder for regulation to require changes in browser though, and then there's legacy browsers, so I can see how we ended up here)