Because it's somewhat like the civil service in Canada. In the US, government officials generally all get canned when the leadership changes; not so in Canada. The staff working in a ministry aren't political. They don't change with the government, and the existing government can't fire them or control their actions directly. This provides stability, since the ministries don't swing nearly as wildly between administrations.
The federal reserve bank in the US works in much the same way, though with a different execution. It has to be separate from the federal government, so that it doesn't potentially wildly shift every 4 years. That would cause chaos with the money supply, and that would eventually lead to the collapse of the dollar's value, as it ceases to be a trusted currency.