You're vastly underestimating the computing market. Startups/tech funding are a microscopic part of the industry. There are millions of small businesses and thousands of major corporations that are slowly moving into the cloud which are all new customers for the taking.

AWS has a head start and a great product catalog, Azure has the windows/office/server ecosystem, but Google has been doing scaled computing for a very long time and has contributed to many of the innovations in the industry. Although they're newer with less features, what they do have is more advanced and the Google name carries a lot of weight and reputation.

Most corporations are still focused on just moving over to IaaS with compliance and security first and Google has done a great job building a solid foundation for this which should pay dividends in the future.