You mean like all the transistors in a single-core processor run in parallel?

Transistors are much less complex than neurons; it takes multiple transistors even to implement a single logic gate, and the input-output function for a single neuron is much more complex than a single logic gate. (Some neuroscientists might say that processor cores are less complex than neurons, though I'm not sure I would go that far. I would only say that it would take a small number of neurons--much smaller than the number in the brain--to be equivalent to a processor core.)