Because that's literally in their job description:
> The judicial power shall extend to all cases, in law and equity, arising under this Constitution, the laws of the United States, and treaties made, or which shall be made, under their authority ... --to controversies to which the United States shall be a party;
> The judicial power shall extend to all cases, in law and equity, arising under this Constitution, the laws of the United States, and treaties made, or which shall be made, under their authority ... --to controversies to which the United States shall be a party;
https://www.law.cornell.edu/constitution/articleiii