I think one other major difference between biological brains and computers, beyond the fact of having a body vs not, is that biology is analog and not digital. A signal going from cell to cell in a brain or up a spinal cord isn't just "on" or "off", but in any number of infinite states. No matter how many bits you manage to pack into a digital "brain," that's a fundamental difference.
Now I can't predict out to the future and say something like digital intelligence is "impossible" or that in the vast universe all life is analog. But from the vantage point of today and what we currently know, it feels that reality is fundamentally analog and so any digital brain can only ever hope to be a simulation of intelligence.
On the flip side, I think we vastly underestimate the various ways intelligence does and can exist amongst living creatures.