Because we imagine that we are what humanity was divinely destined to become, we assume that our prehistoric ancestors were trying to be us, but just lacked the tools and techniques to succeed. We invest our ancestors with our own predelictions in what seem to us primitive and unevolved forms. As an example of all this, we take it for granted that our religions represent humanity's ultimate and highest spiritual development and expect to find among our ancestors only crude, fumbling harbingers of these religions. We certainly don't expect to find robust, fully developed religions whose expressions are entirely different from ours.
The God of revealed religions - and by this I mean religions like yours - is a profoundly inarticulate God. No matter how many times he tries, he can't make himself clearly or completely understood. He speaks for centuries to the Jews, but fails to make himself understood. At last he sends his only-begotten son, and his son can't seem to do any better.