Google started work on ChromeOS before Android became successful in the market and long before it arrived on tablets. Additionally, I don't think Google really wants everybody on an OS that runs native apps. Their speciality is the web. Their money comes from the web and the more time people spend searching and browsing the more money they make. ChromeOS is probably what they wish everybody were using, but they aren't going to ignore Android's success.

They'll keep building ChromeOS to hedge their bets. They can't risk getting shoved out of the OS game in any way.