It’s a bit of both.

It’s not possible to rely on browser controls as-is, because they do not differentiate between necessary and optional cookies.

Browser vendors could agree standards and implement them, exposing these to users and advertisers in a friendly way.

But they haven’t shown any interest in doing this.

I wonder why?

Without laws forcing companies to properly declare which cookies are "necessary", this control you imagine does nothing, as every company simply sets their advertising cookies as "necessary"

One of the hundreds of reasons do_not_track failed. You cannot do something that trusts the website operators, because they are egregiously untrustworthy.

The cookie banner everyone keeps bitching about is a direct example of this. No website is required to have a cookie banner. They choose to, because they know most users click "Yes to all", and then complain about the regulators, instead of the assholes asking you to consent to sharing your data with nearly a thousand third parties

And "browser vendors" will never do anything, because 90% of the market is a literal advertising behemoth, the rest of the market is owned by a company that makes money only when you do things not through the web browser.