The short and sassy answer is because it is their job, not ours.
God is our creator and we belong to him. Jesus is our redeemer and came to earth to free us from our sins. Jesus forgives us for all our sins if we are truly sorry for them. If Jesus forgives us, then there is nothing for us to be judged on. We believe that on the day we are all resurrected from the dead, we will be judged by God to determine who will live with him in the glory of heaven and who will spend eternity in hell.
We do not have the wisdom to make such judgements. We do not know the hearts and minds of people, and neither do we know all their circumstances. Therefore, we can only make judgements based on partial truth.