People tend to forget that Google is a services company. Android OEMs make all the money on the hardware front and Android is free, so Google makes nothing on that either. What they do make money on is serving ads to the people who use their services. They've developed Android primarily to maximize mobile adoption, with their own services at the helm.
The more people use their services - the better. Hence, they only stand to benefit from having those services across multiple platforms.
Lets be blunt. Time and time again its been shown that Android users don't surf the web, don't buy products using their phone, don't buy software using their phone, don't do anything really with their phone anywhere near the extent that IOS users do. If Google as a service company wants people to use their services they have to chase the customers who actually spend money, and currently that is IOS users by a very very wide margin.
Android is almost like giving away free menus to the homeless, whats the point they are not going to come in and buy anything no matter how many you give away. So saying that you have X many "customers" based on the number of menus you have given away becomes meaningless when it comes to how much money you actually make.
Apple makes money from the hardware, from the carriers, from the users who do buy significant amount of Apps, Music and services, and google knows this and is desperate to get into that REAL revenue stream, not the imaginary one that Android provides.