Then it wouldn’t be able to be used to track you across the web.
Until all your data ends up being re-associated with you in some way across multiple websites. Nothing stopping these sites from sharing data if it ends up mutually beneficial.
Intentionally sharing data that identifies users would hit GDPR pretty quickly, no?
Yep, but it's completely invisible to users so that could be tough to prove without a data breach. And there's massive profit motive to get away with this. Right now Google still keeps these websites in check with regards to Analytics/Ads data usage, remove the middleman and things are going to get much worse.
GDPR only safeguards your data from the honest. What we need is a technological solution.
It wouldn't because that's a drastically more complex solution.
If you already have a web server serving your content, setting up a proxy to GA is trivial.
Let's put it another way: 95% of webdevs can drop a code snippet with GA on their page, while only hm.. 5% can set up that GA proxy.