But training is not deterministic. The same training run on the same input can result in very different models. And even if you ignore the intrinsically stochastic part of training, the effort of creating a model is ad-hoc and involving humans. It is a crafted output, not a compiler output. The pre-training is the closest to being mechanistic, but these models are so many layers of work on top of the pretraining, and those higher layers have people in the loop. They run experiments, tweak things, run experiments again.