I personnaly don't think the brain could be as intelligent if it were only a part of it, like if had 3 senses instead of 5 for example: The emulation of a sense by the others might be necessary to bootstrap the system. So we might just be on the wrong path when we try to teach a small machine to learn.

And maybe, once you have a fully functional brain, that it constantly derives out of bounds. Like sudden overheating, which is litterally the step #1 of a depression in a human. So we might not be able to scale an AI beyond the size of one brain. Apart from clusters obviously, but then you need to sustain civilizations of brains, and civs do collapse every dozen generations.

We might not be materially able to find enough energy to power all of those trials and errors.