The EU did that and people still bitch about GDPR banners on websites.
On top of that it literally defines opt-out as the default state
As for browsers, imagine if world's largest advertising and tracking company that incidentally builds world's dominant browser and dominates all web standards would implement this as a browser switch instead of inventing new ways of tricking you into surveillance? https://x.com/dmitriid/status/1908951546869498085 and https://x.com/dmitriid/status/1664682689591377923
Most sites dont even need those popups, but its easier to just shove one on your site than try to understand the specific situations which do need it.