The problem is that even one individual neuron is really complicated. The model we imagine of how a neuron works is very simplified. Useful for some things, but not for simulating a worm's brain.

We know the connections and the neurons but we don’t know the connection strengths and the exact neurotransmitter mixes that work on them. This is just the bare minimum information we need to even try and simulate the system and we don’t have it..