Why did Man make God plural if he dreamed God up?

Okay, first post here.  The first thing I noticed in the very first verse of Genesis.  In English we read "In the beginning God...".   Well, the Hebrew word used here is Elohim.

In Hebrew the "im" ending is like us adding S on the end of a word to make it plural.  Now, I ask you, if man invented God like some say, why did he make God plural.

We read the Bible and see that God is triune, three in one.  We know there is the Father, Son and Holy Spirit.

Ponder this for a moment though.  You are going to sit down and write out history.  More than likely history that has been passed down verbally for generations.  You want to tell the story of creation. Now, if you were going to dream up a god would you come up with one that is not singular?  The whole idea of a triune God is too hard to comprehend for those of us who believe in a triune God.

Others may have already thought of this but I've read the Bible so often and not given this heavy thought.  While struggling through translating I have LOTS of time to think about each scripture so I think of things that I hadn't pondered in depth before.