Oh no on the contrary I think it is a very simple concept.It's not a simple concept at all. It's only simple because you don't want to delve into it.
You expel every single paranormal and "external" factor in the universe and naturally this concept is what you will arrive at as it is then the only thing that makes sense without those factors.
In other words, drop your beliefs about things you can't even see or prove and what do you end up with? A real physical world. There is no alternative that I know of and I beg anyone to show me if there is. Perhaps with the exception of quantum physics and the element of randomness, that is the only uncertainty in this whole equation.
And I don't think it takes much delving into to understand how an entirely physical world automatically means fate without free will.