They know that only a fraction of the user base will ever use it (because they certainly won't ship with it, anymore than Firefox does). They know people shutting off ads aren't clicking anyhow. They know that the people savvy enough to use the ad blocker are also the leading edge people who recommend browsers to other people.

Finally... how would they block ad blockers, even if they wanted to? Ad-blocking proxies are just as feasible as ever. And Chrome is supposed to be open source.

Conclusion: Instead of fighting what you can't fight, roll with it, get the leading edge people on your side, then cash in on the marketshare of the masses who don't block ads.

It's not really that mysterious... unless you're trapped in first-order thinking.