I'm not GP, but the brain is a fantastically complex machine at the cellular level. It's not just electrical activity in neurons, there are also chemical neurotransmitters that play a significant role in emotion, memory, and so on. We understand more about them than we did a hundred years ago, but there's still so much about them where our understanding is limited: for example, we still don't know how the brain produces consciousness (and, in fact, what "consciousness" actually is is widely disputed). But it's pretty indisputable that the brain is an essential component of consciousness: getting your brain knocked around inside your skull (a concussion) usually produces unconsciousness (and sometimes worse), for example.

Now, maybe consciousness is produced exclusively through the electrical action of neurons, in which case computers have the necessary machinery to simulate it. But if neurotransmitters, or other things that we haven't even discovered yet, are vital parts of the process that produces consciousness (and I'm not the only one to strongly suspect they are), then computer hardware, lacking anything that even remotely resembles the action of neurotransmitter chemicals, will be unable to produce consciousness.