It's in the constitution. Article I, Section 8, clause 8:
> The Congress shall have Power [...] To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;
https://www.archives.gov/founding-docs/constitution-transcri...
Unless your question was more, "why was that written into the constitution". In which case the answer basically boils down to the fact that the framers intended for Congress to be the most powerful branch. The modern de facto running of the country places far more power under the executive than the framers intended.
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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