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.