Why did the founding fathers agree to share powers with the states?

The states were in existence before the federal government, and in fact had to ratify the Constitution before it could take effect. Since the states were all essentially sovereign, independent nations, the Founding Fathers could not have done anything without their agreement. In order to get them to accept the Constitution, the state governments of course had to share power with a limited federal government; otherwise they would never have ratified the Constitution, and the United States would not exist.