Google has an obvious incentive to make life hard for ad blocking extension developers. Chrome and chromium exist because their business model is showing ads in it. That's the only reason it exists. Everything else it does is there only to convince users to use it and thus get exposed to ads and tracking. That includes performance work, UX work, etc. that Google puts a lot of time and money in. It makes Chrome really nice to use.
Mozilla and Firefox exists because it's developers wanted to create the best browser possible. That's why it had extensions before Chrome was even a thing and that's why ad blocking extensions exist almost as long as extensions have been a thing. Adblock emerged somewhere around 2002 which was very soon after the first OSS releases by Mozilla reached the 1.0 stage. Also Phoenix, Firefox' ancestor became a thing around that time. Tabs and extensions were some of the early things that made that popular. Ad blocking always was the #1 use case for extensions.
Ironically, that's why Chrome has extensions. The only reason it supports extensions is that not having that would have made it impossible to grab market share from Firefox (and Internet Explorer). Having support for both extensions and ad blocking were a hard requirement for Google despite its business model. But now that it is the dominant browser, that feature is no longer as important as it once was. So, Google has been slowly making it harder for extensions to interfere with their ads and tracking. They can't make it too hard or their market share will evaporate. But they don't have to be particularly good at it. And now that they have Android, they don't have to worry as much about losing market share. Android exists for the exact same reason. Chrome is losing relevance as a revenue stream as users consume more content on Android via "native" apps running in a virtual machine compiled against mandatory proprietary ad & tracking technology.