Most part of the remaining brain is used for muscle control/senses and other bodily stuff that LLMs don't have.
LLMs are not brains but tools. They still can do logic and vastly outperform our brains in terms of memory volume, speed, they can work 24/7, easily replicable, scalable, teachable.
To make an LLM 2x more powerful will take us years. Brains will hopefully do that in several million years.