They didn't think it - they clearly stated that rights come from God, not man. America is a nation ruled by LAWS, not men. All other nations are nations of men. Many people live as if moral absolutes exist -I am one of them and there are many like me. Just because no one can live perfectly according to the 10 commandments does not mean the 10 commandments are not universal moral laws - they certainly are.
You hit on the very heart of the matter, however. Paul the Apostle told us all what the ultimate purpose of God's laws are: They are to show us how sinful we are and that we have no righteousness of our own and that we cannot deserve or earn our way into heaven. The moral law is a "schoolmaster" to drive us to Christ. Therefore, we must have the covering of the righteousness of Christ as He lived the perfect life for us and when we put our faith in Him, His righteousness is imputed to us. So, your point is true in that no one on earth can live up to the 10 commandments - it's impossible. But I love God's moral laws. Why you may ask? Because they are an extension of God's good, pure and holy character - they flow from His very person, and I love God. God is pure and holy, we aren't, so we need Christ. Without Christ we will be judged for breaking God's moral laws. He does not want to punish us but He must because He cannot go against His own nature, which demands Perfect Justice. By that standard, we are guilty.
Man has been taking things from other men throughout history. It's what we do. That is why I do not believe there are moral absolutes. As long as man has freewill, he cannot practice moral absolutes. Obviously, our definitions of moral absolutes are very different. As an example, I think of an absolute as something which is pure or complete, that which CANNOT be modified, something unconditional. I don't believe that this definition can be applied to morality.
Your definition of absolutes is correct. Man can practice moral absolutes if he loves God and wants to please God out of gratitude, but man cannot obey them perfectly. Why? Because all men are fallen beings, sinners. The bible says that "the human heart is desparately wicked, who can know it?"