Here's one: neurons are cells, and cells are really complex and have all sorts of internal state on different scales (organelles, molecules, etc). Neural networ

Here's one: neurons are cells, and cells are really complex and have all sorts of internal state on different scales (organelles, molecules, etc). Neural network models in computers are an extremely simplified abstraction by comparison. And that's still ignoring all the other things that may or may not be happening in the brain beyond the connections betweens neurons themselves.