The origin is that the CIA came into existence when the FBI/J Edgar Hoover were at their height of their influence. The FBI, military, and state department had

The origin is that the CIA came into existence when the FBI/J Edgar Hoover were at their height of their influence. The FBI, military, and state department had an effective veto over new powers for the CIA, so the CIA agreed to lots of rules to make them happy: no overlapping function, no recruiting current employees, etc. The argument was: “combining domestic and foreign intelligence is something the Nazis and Communists do, so it’s bad.” There have evolved good reasons for the distinctions. The CIA never engages in criminal prosecution, so doesn’t need to worry about revealing evidence as part of a prosecution. FBI agents, by contrast, are paranoid about what information they are exposed to, for fear it could come out in court. That’s much more recent, and would have been a surprise to Hoover. In fact, Hoover used the FBI to filter information gleaned from the Vernona Project. The president would receive summaries of analysts reports that were simply “vouched for” by Hoover. By contrast the DEA was formed during an era when federal law enforcement and intelligence were at a low point in influence, so they stole people, mission, and influence. That’s why the DEA has its own foreign intelligence, domestic intelligence, diplomatic corps, and even judicial system.