That's likely not the real rationale. If people browse with Chrome, then google is the default. That is immensely valuable to google, as google's payments to apple, mozilla and android manufacturer's show.

In theory Chrome could exist as an independent business if it were allowed to take bids for search default from google. But if the US govt broke up google they would likely also ban the sort of deal that would let Chrome be a viable business on its own.

By the time Chrome came out Google already was in the position where everyone knew to set their default homepage to Google in IE in the same way they automatically go to install Chrome now.

I'm surprised no one has mentioned the pseudo-control Chrome gives them over web standards. They can implement experimental APIs in Chrome and immediately use them in their webapps.

Correct. Google's real concern was not that people wouldn't use them as their favorite search engine; it was that relying on Apple and Microsoft to be stewards of access to the web was a huge business risk because, hypothetically, if Apple or Microsoft decided to block google.com at the browser level (or pick your favorite equivalent scenario, like failing to implement a standard that Google absolutely was going to be relying upon to provide service), Google was screwed.

Chrome was a business risk mitigation move.