After thousands of years of research we still don’t fully understand how humans do it, so what reason (besides a sort of naked techno-optimism) is there to believe we will ever be able to replicate the behavior in machines?
The Church-Turing thesis comes to mind. It would at least suggest that humans aren’t capable of doing anything computationally beyond what can be instantiated in software and hardware.
But sure, instantiating these capabilities in hardware and software are beyond our current abilities. It seems likely that it is possible though, even if we don’t know how to do it yet.
That humans come in various degrees of competence at this rather than an, ahem, boolean have/don't have; plus how we can already do a bad approximation of it, in a field whose rapid improvements hint that there is still a lot of low-hanging fruit, is a reason for techno-optimism.
Thousands of years?
We've only had the tech to be able to research this in some technical depth for a few decades (both scale of computation and genetics / imaging techniques).
Well, understanding how it works is not a prerequisite to being able to do it.
People have been doing thigs millenia before they understood them. Did primitive people understood the mechanism behind which certain medicinal plants worked in the body, or just saw that when they e.g. boil them and consume them they have a certain effect?