A large part of it is that the publisher is having to make code changes to do so, and as part of doing so, may as well set, and then forward cookies to your adtech company right? Assuming you're not in a GPDR country.

Helps your advertiser still gain some info if the user is blocking the 3rd party cookies they'll try to render anyway.

The drive to map 1st party cookies to 3rd party cookies to compensate for stricter default cookie blocking in non-Chrome browsers, and the generalised failure of doing so, is what drove Google's FLoC.

Even though user targeted ads are generally pretty ineffective, advertisers are addicted to them.