The main difference is computer technology is designed to be very general purpose. The brain is more like an ASIC that's hardwired to run one specific algorithm. GPUs are also computing 16 or more bits of precision, when real neurons are very low precision. There are some other differences, like how real brains are incredibly sparse and most of the synapses at any given time are dormant and not using much energy. They are also very sparsely connected to each other. While our current NNs are very dense and need to spend energy to compute every single connection each cycle.