The Constitutional power of Congress is to enact laws, but not to execute them. Article I generally lists other authorities of Congress, e.g., "to coin money", but the Treasury is an Administrative office, not a Legislative one. Another interesting example would be the establishment of a Post Office, which is now an independent agency, though under the Executive.

You'll also find: "The executive Power shall be vested in a President of the United States of America." Art II, Sec 1. That establishes the Separation concept, though neither "separation of powers" nor "checks and balances" are explicitly stated in the US Constitution. They are part of the political discussion in which the Constitution was framed, however.

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