Yes, and right now brains are a bunch of nerve cell bundles that may well be basically equivalent to matrix multiplications and activation functions.

But a human brain has 80 billion neurons, each with around 10 000 synapses. That's 8 x 10^14 weights. It's probably noisy and maybe it's equivalent to ANNs that are smaller than this, but even if each synapse was only 1/100 of a bit on average, it's still 8 x 10^11 bits, i.e. 800 GB just for the weights. That's not going to fit on a graphics card.