There's no network effect from having the Uber app. All Google would need to do is push a notification to every Android phone that has Google Maps (so, all of them) that says "book your next trip through Google maps and the first ten rides are free!" and they're in business.
I mean, I use mostly Lyft in the US. And if there were another app or two that had a reputation for being cheaper or better service even if they didn't service everywhere, I'd be installing that in a minute. Brand has some value and I don't want to be installing one or more apps for every city I visit. But it mostly works when someone is as good or better for most purposes.
