I wonder why ad tech can‘t be „Let‘s show ads that correspond to what‘s being talked about on that website.“ Kinda like what Google suggested with Topics but without following me through the internet.

They could be. Sites could talk directly to advertisers, and put the ad directly into the page itself instead of asking the ad server for a random ad. Most ad blockers probably wouldn't notice it because it's part of the actual page.

But then they'd lose out on the tracking data and would be responsible to make sure the ad doesn't annoy the shit out of you, so they're not going to do that.