system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What's the core differences between the CIA, the NSA, the ATF, The FBI, the Secret Service and Homeland Security?

Why can't they be combined into one agency?  All seem to do pretty much the same thing.  I'm confused as to what each of them can and can't do and the jurisdiction of each agency.

*update*  Thank you everyone for kind, thoughtful answers.  I had a general idea of what each agency was supposed to do and assume they all work together as needed, just not the fine points/jurisdictions.
gpt
They all have very different charters and are tasked with very different activities

FBI:  detect and prosecute crimes against the United States

CIA:  Counterterrorism, nonproliferation of WMD, counterintelegence, cyber intelegence and forgien intellegence gathering

NSA:  global monitoring, collection, decoding, translation and analysis of information and data for foreign intelligence 

SS:  Prevention and investigation of counterfeiting of U.S. currency and the safety of current and former national leaders and their families

Don't believe what you see in the movies - they each have a very specific charter and duties.  They are also all very large organizations with complex managment structures to control the power that the agencies wield.