**Why do we need to install an app to get push notifications?**
We are used to get pre-installed solutions on Google-certified devices for many services such as the browser (Chrome), the map (Google Map), keyboard (Google keyboard), camera (Google Camera), push notifications (Play Services), etc.
Our free solutions aren't pre-installed on our devices: if we want a free map, we need to install it. It works the same for the push notification service.