I'm trying to make youtube app clone and i'm using youtube api to do so. But using youtube api requires official app to be installed on the device. Why is it officially necessary to use the api?
The API client library interacts with a service that is distributed as a part of the YouTube app for the Android platform
From the doc. The API itself is just a very small wrapper to use the existing service used by the YouTube app, it doesn't have all the necessary code to play the video.