[thesuperbigfrog]

>> I think this kind of computer exists and is called ”brain”.

True, but there are ethical and rights considerations unless you are building / growing an artifical brain.

You can't just grab a human or sufficiently-suitable animal brain and turn them into your personal mentat (https://en.wikipedia.org/w/index.php?title=Organizations_of_...) or computing servitor (https://warhammer40k.fandom.com/wiki/Servitor) to do your computing for you. (At least, not ethically.)

Where would you get a brain to train?

Also, it's not easy to train human / animal brains in their natural state as any parent / animal trainer can attest.