The sites that display ads, wants to make sure ad blockers can't block them.
Currently both sides adapt, if the way it works is locked down ad blockers quickly will become obsolete.
For example a whole ago most sites were creating popups with ads, after it became bad, browsers started blocking popups, initially by only displaying then when user actively clicks. So sites started opening a popup when user made a first click anywhere on the page, eventually browsers started blocking all popups and just notifying user that popup was triggered, giving them choice whether they want to see it.
This solved the old pop-ups, but because of that a new popups were created, that use CSS to show it within browser window covering the text. In addition to that, the CSS layers were used to implement other attention grabbing mechanisms, like ad that stays in place even when you scroll, or suddenly ad appears between text etc.
This was targeted by ad blockers, which constantly adapt. Most ads are served from different domains, since typically the ad content is provided by different company, but increasingly we see ads being served from the same domain as rest of the website, or website randomizes CSS component id etc.
What chromium authors are doing is to instead provide API for ad blockers to use and list their rules and let the browser do the blocking. Supposedly that's to improve performance. The problem with it is that it will essentially fix adblockers in one place, they no longer will be able to adapt, eventually new kind of ads will start to show up, that adblockers won't be able to block. And now with other changes that Google successfully pushed, such as https everywhere, http/2 and http/3 is nearly impossible to block ads through a proxy.
This is why I personally prefer to stick with a Firefox.