The properly GDPR compliant cookie banners allow you to itemize certain items on the website's TOS that you may choose to accept or not accept. Website TOS are very useful for the company operating the website and the cookie rules allow you as a visitor to get some of that usefulness back.
Browsers like Firefox could hypothetically kill cookies in their browser tomorrow but doesn't. Or at least make a big stink about it. Do you think they should? Do you think we'd be better off as a society?
I think when you click "accept", you also accept to things like fingerprinting and storing a fingerprinted identity on their server, as well as perhaps supercookies, that allow your ISP to track you.
Browsers can't tell if a cookie is a generic setting ("chose Yes/No on a banner") or a uniquely identifiable one; and they can't tell if a cookie is functionally required (ID for a logged-in session) or not (ID to track random visitors).
The distinction is legal, not technical; so it has to be enforced by legal, not technical means.
Firefox COULD default to cookies off (with an in menu widget to force them on for non-automatic handling), and if any forum submission happens _ask_ if the end user wants to accept the site's cookies.
Looking at a typical site, a reasonable user might want to accept one (or perhaps a couple) of many dozens of cookies a site attempts to set. Choosing it manually per site per cookie is difficult but perhaps theoretically possible, however even that still requires cooperation from the site to honestly identify that this one is the cookie which is functionally required, and these fifty are for ad tracking, and ensuring that cooperation still requires legal means and can't be done with purely technical ones.
Furthermore, there is the important distinction about multiple uses of the same data. There are uniquely identifiable cookies that are functionally required for one purpose but the site may want to use it for other purposes as well (e.g. share that data with heir "trusted partners" for targeted advertising) for which user may reasonably want to refuse permission, so a browser accepting a cookie doesn't imply such permission and something extra is required.
