Well, you can’t audit the proprietary hardware drivers most android manufacturers use or the many proprietary apps from google etc that sit on top of android or the changes that the carriers make.

I remember when I was an Android developer dealing with several issues relating to the fact that one carrier put a proxy in the networking stack.

Here is a recent example:

https://www.theregister.co.uk/2016/11/15/android_phoning_hom...

You can audit iOS. Security researchers do it all day long.