The thing is, this process is so complex it’s useless to talk about determinism of human actions.

I would agree, at this point in time it isn't really possible for us to truly understand the overall system that affects how we make decisions as in. We can at least start with the basics of understanding that it exists rather than stating that we somehow have this sort of magical "free will"

A silver atom's angular momentum is aligned based on the field in a Stern-Gerlach device. This alignment determines how these atoms react to a future magnetic field. Therefore a silver atom's angular momentum is deterministic. But it's not.

A totally deterministic universe would have been a lot simpler. Why go through so much trouble?

Why there should be a why?

You don't get to know in advance which questions will pan out into answerable ones. You could answer "why should there be a why" to literally and question about why. In fact, you could have said that about the motion of the planets, right up until Newton - which shows that the category of things that can be answered like that contracts over time.

That's a how, not a why. Asking why is it worth the trouble ascribes intentionality to the universe where none is expected