Because they're part of the US government. They're "independent" in the sense that they're not as closely tied to the head of the executive branch as e.g. cabin

Because they're part of the US government. They're "independent" in the sense that they're not as closely tied to the head of the executive branch as e.g. cabinet departments, but they're still answerable to the judiciary at the very least. This is a fairly strong argument that you've made, but I wonder if it was the one you intended to make.