Because
"[the brain] is sending signals at a millionth the speed of light, firing at 100 Hz, and even in heat dissipation [...] 50000 times the thermodynamic minimum energy expenditure per binary swtich operation"
https://www.youtube.com/watch?v=EUjc1WuyPT8&t=3320s
AI will be quantitatively smarter because it'll be able to think over 10000 times faster (arbitrary conservative lower bound) and it will be qualitatively smarter because its software will be built by an algoirthm far better than evolution
It doesn't really matter whether the AI uses their full computational capacity. If the AI has a 100000 times larger capacity (which is again a conservative lower bound) and it only uses 1% of it, it will still be 1000 as smart as the human's full capacity.
AGI's algorithm will be better, because it has instant access to more facts than any human has time to memorize, and it will not have all of the biases that humans have. The entire point of the sequences is to list dozens of ways that the human brain reliably fails.