Because a human brain is not field programmable, and improving cognitive ability isn't as simple as plugging in another stick of RAM. The worry about AI is the bootstrap scenario, where an AI gains sufficient intelligence to modify and upgrade itself.

Sure, you're right in that humans don't understand enough about our own brains to perform such modifications, but that may simply be down to a lack of knowledge, rather than not having sufficient cognitive ability to comprehend the brain.

Additionally the hardware which an AI can run on is generally assumed to be significantly more malleable than our limited gray matter confined within our cranium