Because Google isn't a mobile company - they're a service company. They don't care if you use Android (which makes up barely any of their revenue) or iOS, so long as you use their services.
Google makes money by connecting their users to commerce opportunities. Their ability to monetize is linearly related to how many active users they have. They'd rather have 70% of the market through Android plus some proportion of non-Android users finding them through apps than have 75% of the total market and rely 100% on Android to attract users on mobile.
They get more total users and have a more diverse base. If Android starts to lose share, they still have the ability to monetize the other platforms.
Google makes money by connecting their users to commerce opportunities. Their ability to monetize is linearly related to how many active users they have. They'd rather have 70% of the market through Android plus some proportion of non-Android users finding them through apps than have 75% of the total market and rely 100% on Android to attract users on mobile.
They get more total users and have a more diverse base. If Android starts to lose share, they still have the ability to monetize the other platforms.