At first it sounds insane that Google would do such a thing since while ad blocking is growing, enabling the feature natively especially by default would incredibly increase the number of ads blocked. My guess is, that where other blockers by default can easily block all google ads, Chrome blocker would not block Google Ads because it would classify them as acceptable. And Google would then hope that people would use…
So if it's any good at blocking Google ads, it costs them money.
If it blocks ads other than Google ones well, and allows Google ones to display, then they run the risk of lawsuits or antitrust action.