Why does the human brain not overfit when training at some task?
I would say the human brain overfits all the time! Gambling addiction,  superstition and anxiety disorders are all examples of overfitting. We are optimized for seeking patterns and avoiding threats. Our brains mess this up all the time! But having said that, one of the major differences between the human brain and a neural network is the amount of information that's being supplied. Neural networks overfit in part because of limited data sets. Our brains have nearly unlimited data sets available, and this provides us at least the opportunity for some self correction.