Requiring an exact justification for a specific NN architecture is not productive. Simply put, such justification almost never exists, yet NN clearly out-perform simple polynomials in a wide variety of tasks. Even if you know that a NN performs better than a simple polynomial on a task, you're very unlikely to get an exact explanation on why a specific NN was chosen vs. the infinite number of other possible NNs. If you require such an explanation, then you're going to miss out on a lot of performance.