A machine learning model is a computer program. It is fundamentally a math equation, which we understand completely.
Lol you couldn't be more wrong about this. One of the most widely commented things about DNNs is that we don't really understand them completely. I don't know how you would miss that if you knew anything about AI at all.
A living brain is not fundamentally a math equation
It is. A very complicated one, sure. Which part of the brain do you think is impossible to simulate with maths?