Complexity cuts both ways. Complexity, as every programmer knows, is no friend, and can easily create more problems than it solves.

Often the most efficient solution to a problem ends up being a simple one. It would be very surprising if the human brain's Rube Goldberg machine was anywhere near close to a mathematically optimal implementation of intelligence.

It's possible that much of the complexity in a biological neuron is simply working around other complexity introduced by biology, solving problems that we programmers do not even have to think about because we can simply directly use matrix multiplication.