That might bring Google afoul of various Monopoly Commissions. Also, such a service couldn't be advertised as 'internet'. Thirdly, Google make money from people visiting the sites of companies who advertise on Google; an advert for BobsCarRental.com would be useless if the user couldn't visit it. Lastly, people remember the horror of walled gardens like AOL and CompuServe.
Other than that, you plan's a good un!