Because Google is trying to add internet-wide drm which would make ad-blocking difficult, if not impossible, and they have a large enough marketshare to potentially force it.
Tbh I don't think that's totally outside the realm of possibility as it takes control out of Apple's hands. However, I'm not sure if Apple's userbase is really large enough to stop Google.
Supposedly Safari makes up about 20% of global browser usage (chrome is at 63%, Firefox is about 2.8%) which could be large enough to stop Google, but that's also assuming that Apple won't cave to users and/or corporations complaining about how their site doesn't work on Safari. Apple might even create their own alternative system that lets them maintain control while still providing drm.
Google wants to implement DRM on websites. This means the websites could block any browser that doesn't implement this feature. And this feature would make it impossible (or at least really hard) to block out ads.