Both (separately though, the two answers are not related even though I put them in one post - I'm allowed to have more than one thought about something).

We just don't understand intelligence (not the brain, intelligence) well enough to duplicate it, never mind improve it.

And I don't think we ever will because to understand something complex you have to be more complex than that thing, but by definition we can not be more complex than ourself - so understanding our own intelligence will always be beyond us.

That's the objective part.

The subjective part is that I personally believe that intelligence is not physical. The brain mediates it, but the source is elsewhere.

The soul maybe, or that we are really game characters in an MMOG.