I still remember the day, when the browser would show you a pop-up "Allow somesite.com to set a cookie? Yes - No"... Something like that (with a "remember this choice" checkbox) would be so much more economical, than making every single website implement it individually. When the browser does it, then websites also can't ignore it, cheat it, or use dark patterns to avoid it.
The big misconception is that the consent forms are only about cookies. They aren't - they are about data collection and processing in general, no matter the technical means. It could be cookies, but it could be IP addresses, browser fingerprinting, or information you manually entered for a specific purpose (delivery address to receive a package) and don't want to be reused for other purposes such as marketing or tracking. None of that can be communicated at the browser level unless you want to have a gigantic spec that encodes every possible use of personal data imaginable.