It's a two sided market that takes a significant investment to create. Yes, Google can add G-Car to maps, but where are the drivers coming from? They're not going to switch unless they can expect more income than from Lyft/Uber. Google can't provide that without investing a lot of money in incentives.
The best analog to Lyft/Uber is Mastercard/Visa. It's theoretically easy to enter the market, but in practice it's going to be very difficult to develop your own two sided marketplace going against the incumbent competitors.
Or indeed Google develops a competitor themselves if Uber and Lyft gain 9-digit valuations. It could do that in less than a year and for a few hundred millions at most. Taking a page out of Apple's book of vertical integration and squashing popular services with in-house offerings.
That assumes that the feature in Maps not only exists but is widely used by consumers in preference to individual service apps; my impression is that that is not really the case.