That would be more effort than including a single html-script tag to import google analytics. I have hope that most parties would decide that the extra server load and difficulties would make it not worth it.
You underestimate the desire for precision tracking, unfortunately. Hiding behind custom subdomains is common.
Stepping up to cloaking it to be delivered from the application is more effort but it'll happen.
You overestimate the technical ability of publishers. Frankly, it’s pathetic to rely on a third party’s hot linked JavaScript, but no one can be arsed to understand how it works, so they just add the tags to GTM instead of realizing that they could trivially implement A/B testing or whatever themselves.