These agencies work with delegated powers. It is completely impossible for such a limited number of people as the American Congress to be experts on everything. They need advisors and structures to help them understand the world and make the right decisions, but also to make sure that these decisions are enforced.
This may not be fully developed in the US constitution because the world was much simpler back then, but it is entirely compatible with it.
This may not be fully developed in the US constitution because the world was much simpler back then, but it is entirely compatible with it.