It surprises me that more people don't just block third-party cookies. No need to maintain blacklists that way. Sure, you still see ads, but it keeps tracking at bay.
If third-party scripts or images are still being requested, then what prevents a site- and/or page-specific identifier from being included in the URLs of those assets? And what then prevents those site IDs, page IDs, the IP address you used to make the requests, your user agent's User-Agent string, the Referer header values, and other identifying information from being collected and stored? What prevents them from them from then tying this information together in a way that tracks you and your browsing behavior?
Keep in mind that cookies of any type are not at all necessary for such tracking to occur. The only way to truly avoid that kind of tracking is to not make any such requests in the first place.
The change is extremely simple, and the data loss from people who block third-party cookies is like 5%.
So what prevents sites from deploying these technologies? Economics. To your point, there are no insolvable technical barriers. But right now the marginal return return on these technologies in no way stacks up to the marginal effort required to deploy them. Considering the fraction of the population that even knows what a cookie even is, I do not anticipate this changing in the near future. If everyone starts blocking third-party cookies, then yes we'll see an uptick in use, but I don't see that happening this decade.
I still get shown ads correlating to other sites I've visited, searches I've done, or emails I've received on Gmail.
I searched for Niles and Russound multi-room audio one Friday last summer, and HTD.com (Home Theater Direct) has been showing me whole house amps everywhere I go ever since.
The third party cookie block doesn't seem to help much any more.
Google whitelists their own third-party cookies by default in Chrome and it's possible other browsers come with a preset whitelist as well, maybe that's what you encountered.