Where do people think the agency derives 100% of it's power from? The President!
You clearly do not understand how the American system of governance is supposed to work. Every department has enabling legislation. Below is a link to a law passed by Congress and signed by the President establishing the DOJ.
Congress can design an agency, set it's processes and procedures, and give it money to run.
But the President is in charge of it within those bounds. The President can't decided DOJ doesn't need an Attorney General (Congress already decided that), but the President gets to pick the AG and the AG reports to the President.
You clearly do not understand how the American system of governance is supposed to work. Every department has enabling legislation. Below is a link to a law passed by Congress and signed by the President establishing the DOJ.
https://www.justice.gov/sites/default/files/jmd/legacy/2013/...