Human beings are limited, fallible, and corruptible. The Federal Government and the States, themselves comprised of human beings, has no power and no right to guide the progress of human evolution.
This is what liberals cannot understand and the founding fathers did. Until God himself -- with his infinite wisdom and love and power to create matter out of energy -- takes over the planet, government by frail human beings will never bring about a perfect world. So, you NEED checks and balances.