Let me try to explain another way. Lets say your brain had 10 neurons... Well, then you certainly could understand it. What if it had 1000? Sure... You could probably handle that. Ok, but the human brain has 100 billion neurons. Where are you storing that knowledge?

The good news is that there is a lot of evidence that computation in the brain is more modular than that... It's not necessary to understand each neuron... Just like it's not necessary to understand each artificial neuron in a large nueral network.