Because they tried it, and it was ignored. Anyone who actually worked on browser engines knew it was bullshit that the big internet advertisers (google, Facebo

Because they tried it, and it was ignored. Anyone who actually worked on browser engines knew it was bullshit that the big internet advertisers (google, Facebook, etc) were using to deflect whichever privacy disaster was in the press at the time. The rule required DNT not be enabled by default. It was optional for advertisers to follow it, and it was very clear that if there was any significant population that enabled DNT the advertisers would start ignoring it. And they did. They went even further: they used the DNT state to to track users. Nothing like this on the client side has any value unless it is made illegal to ignore such flags, and that is actually enforced.