Because God is love. Love is expressed best when it is toward something or someone else. So God chose to create a world and people as an expression of His love. He created people so they could love Him in return. He could engage in a relationship between Himself and the people He created and they could express love through each other.
Romans 8:28 -- "And we know that God causes everything to work together[a] for the good of those who love God and are called according to his purpose for them."
God is always working for good. He is always taking care of His creation. He guards and guides us through our struggles so that we can do something amazing do to our relationship, obedience and trust in Him. God can use ordinary people to do amazing things because we are created in His image. We can have faith in Him because He loves us.