That is a power granted to congress, not a constitutional right.

That is, the constitution grants Congress the authority to create a legislative IP right. It does not create a constitutional IP right.

You can tell, because Congress could say: “stuff it, no IP rights for anyone. Copyright no longer exists for new works”, and that would be constitutional.

People interpret the constitution in different ways, but this is commonly interpreted as a requirement not just a power. Critically, this stuff is interpreted by the Supreme Court not simply anyone so someone could bring a case should congress fail to provide any protection.

Thus congress could specify that copyright only lasts for 1 month, but they can’t just ignore it or set it to 1 second without the courts getting involved.