If we are doing as God created us, and He knows all that we are going to do, why do you believe that we are all sinners? As the Creator of all things, didn't God create the outcome of our free-will choices? Even down to our choice of religious beliefs?
Bear in mind that you must believe that the Bible is the inerrant Word of God and use that premise for all other understanding.
We are born into sin because of original sin. When it is said that God created man in His own image, it means that man was created perfect and sinless. But man was tempted and made the choice to sin. All mankind carries the consequences of original sin.
I can't tell you why any better than I can explain why we need sleep, food, light and dark. I accept it as God's choice and omniscience.
I avoid "why" because it doesn't matter to me. I know in the depths of my soul that I am His child and I trust Him for everything that has gone and will come.