But that is a lot harder to do and requires more resources.

If you have a tracking pixel now, the company directly knows your browser from you downloading that pixel. If they were to implement the single-backend stuff, the site would have to gather all that information themselves and then send it to all the trackers. But they can’t just send it somewhere, because then everyone could send bogus info to them so you need verification and an api and that is costly and each company would build their own api so you need to buy a program that speaks all those apis… You get the point. It’s a LOT more work than just pasting the text for some pixel somewhere on your site and let the others do the rest.