Why is Ecclesiastes included in the Christian Bible when it contradicts so much of Jesus' message?

You misunderstand Solomon in Ecclesiastes. Solomon is said to be the wisest man who ever lived and in his search for knowledge Solomon is telling you that your only reward is with God. Solomon is letting you profit from his search of man's knowledge of which today man people hold in high esteem the knowledge of man (science) and forget the far better knowledge of God.