It isn’t. It’s called ארצות הברית (lit. “Countries of the Covenant/Alliance”)—which is indeed a poor translation of “United States”, as many people have remarked over the years.
A far more accurate translation would have been המדינות המאוחדות (Hamdinot Hameuḥadot)—but that’s more of a mouthful, and too close to האומות המאוחדות (HaUmot Hameuḥadot—the United Nations), so the poor translation has stuck.
So if you had any furtive hopes that the Hebrew name of the U.S. harboured an unknown divine approbation, I’m sorry to dash them—but there it is.
Notes by Autumn Light 