Because it won't take off. Right now, the advertisers are basically hoping for you to be too lazy to click around ten minutes to find the 'no'-option. If every user would be presented with a fairly weighted chance once, hardly anyone would click yes. Accepting this standard would undermine their business even more.

That existed. https://en.wikipedia.org/wiki/Do_Not_Track

It failed horribly because it was voluntary. But now that it's a GDPR requirement, perhaps that might have a snowball's chance in hell of succeeding.

No, it failed because it acts on the wrong side. If you don't want to be tracked you shouldn't send cookies in your request.