separation of powers for one. the doj is part of the executive branch as defined in the constitution. sec is a regulatory agency, not law enforcement. they just trump it because thats the way our government works. think about it. you receive a decision from the sec you dont like. who you gonna call? thats right, the Judicial system. Who represents the sec in a criminal, not civil prosecution, thats right again, the DOJ in the US courts. you go bankrupt and need a trustee. who does that trustee work for? the DOJ.
Thats just the way it is.