The only reason this works is because humans built circuits. If their behaviour was too complex, we would not be able to design them to do what we want.

Quite so, but . .

A neuron can use arbitrarily complex calculations, because evolution's only requirement is that it works.

Ultimately this is all we care about as well.

We do simulate circuits at the lowest level now to understand functionality before we try to build it, and as our simulation capacity expands we will be able to handle increasing complex designs and move into the space of analog circuits. Digital ASICS for AGI would probably come well before that, of course.

Really its a question of a funding. Our current designs have tens of billions of industry momentum to support.