Is it because we haven't invented something better than backpropagation yet?

From what I understand, a living neural network learns several orders of magnitude more efficiently than an artificial one.

I'm not sure where that difference comes from. But my brain probably isn't doing back propagation, it's probably doing something very different.

Your brain is doing several different things, because there are different parts of your brain.

(eg different kinds of learning for long-term memory, short-term memory, languages, faces and reflexes.)