The model has multiple layers which are basically a giant non-linear equation to predict the final shaded pixel, I don't see how it's inherently difference from

The model has multiple layers which are basically a giant non-linear equation to predict the final shaded pixel, I don't see how it's inherently difference from a shader outputing a pixel "at once". Correct me if I'm wrong, but I don't see how you can simulate a PBR pixel without doing ANY pbr computation whatsoever. For example one could imagine a very simple program computing sin(x), or a giant multi-layered model that does the same, wouldn't it just be a latent, more-or-less compressed version of sin(x)?