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.
No one who actually works on digital neural networks thinks this. We may not be able to predict the behavior of a particular neural network with certainty (because there are a lot, like millions, of variables), but that does not mean that we don't understand how they work.
Which part of the brain do you think is impossible to simulate with maths?
simulation ≠ reality