That's also only for value they extract from being the default search engine (which probably means they are used by 95+% of users). On Android they can also count on being the default for maps, mail, browser, and app distribution (not to mention that Youtube is usually pre-installed, and that they have been forced to admit that they collect location data from Android devices even when location services are turned off). So they have the opportunity to collect vastly more data and target vastly more advertising.