Because my sins and offenses against a holy God are so completely wicked they require an infinite payment - a payment in sinless blood. A Perfect man had to die in my place to satisfy the justice of a Perfect God, so that the unutterable sinfulness of my soul would be forgiven and a sufficient, saving righteousness would be freely given to me. My moral bankruptcy is so great that I can not even pay a fraction of a penny toward my own salvation. I can bring only the immensity of my debt and the fact of my inability to pay.
Man's iniquity is so vile that there is no way for us to approach God (or even continue to breathe!), apart from the Mediator who experienced the immeasurable spiritual agony we deserve, died in our place, and rose to new life so that He might apply the benefits of His sacrifice to His chosen ones.