I think because they still collect information from your phone and correlate it under your Google Account. I created an account solely to install software (security) updates to the built-in components, and it greedily grabbed my contacts from my OwnCloud instance and sync'ed them to my Google Account. As colordrops mentioned in another reply, they are very sneaky about building a user profile.
And they always upload usage data, how many times you opened which app, how long it stayed in background and how often you used it in foreground, overall use time, etc.
