It should be quite obvious.
Google's main source of revenue is ads. Then they also make money based on cloud subscriptions.
They want to make money.
The more people use their services, the more Google can serve up ads. Ads in Gmail, ads in Google Maps, ads in normal Google searches. You get a certain amount of cloud storage for "free," but if you want 100 GB of Drive storage or want the unlimited streaming of Music, you pay for it (a subscription).
What does it really matter? You can still use the Google Music app to play locally stored music files, and you can still use third-party music players like Poweramp. As long as you have a choice, Google can "push" whatever it wants, and you have no basis for complaints.
One of the major reasons I don't use an iPhone is Apple's insistence on using their apps. Without jailbreaking (which is becoming increasingly less commonly available or just impossible, depending on what iPhone you have or what version of iOS), you can't change your default apps. Got Chrome or Dolphin installed? Too bad. Click a link, and it'll open in Safari. Got Google Maps installed? Too bad. Click an address, and it'll open in Apple Maps.
As long as Android allows me to choose default apps, I don't care what Google wants me to use.