While technically true, in practice it is far from it. Nearly all of the core updates come from Google.
If you forked the Android project then you would no longer be depending on Google for the updates.
A fork is a starting point. The two projects would diverge over time to the point they were no longer compatible and become two different operating systems.
This should scare Google. They could potentially lose control of Android (and thus unable to install spyware on it).
This happened to OpenOffice (which was an open source version of Sun’s StarOffice). It was forked to another open source project: LibreOffice. The two grew apart and LibreOffice has became more popular.