Why it is not possible to play an audio file on a voice call in android

This question might seem to be a repetition of the questions such as following:

How to play an audio file on a voice call in android

Background Audio for a Call in Progress - Possible?

The answers of these questions suggests it not possible to play a pre-recorded audio on a voice call in android. I want to know why it is not possible? What is the limitation (hardware/software)? Is it really a limitation or done purposely? Can we alter the source code of android to make it possible?