Why Penrose thinks General Intelligence is unachievable

It must be achievable otherwise it wouldn't exist. *

This whole argument reminds me of the scientists who claimed in the late 1800s that heavier-than-air flight was impossible, never mind the millions of birds and insects who proved every day that it was possible.

* Just because the technology isn't there YET, does not mean it will NEVER be there. Arthur Clarke called this kind of thinking a 'Failure of Imagination'.

It's difficult to map the subject line about the unachievability of general intelligence to anything in that piece. What am I missing?