Back when nuclear weapons were relevant, every service wanted them. The air force wanted bombers and ground-based missiles. The navy wanted submarine-launched missiles. Even the army got in on it, with nuclear artillery.
Why have we waited until missiles are fading from relevance, years after the end of the cold war, and only now decided they need their own branch?
Basically, most countries need a dedicated org to manage A2AD and Geospatial strategy - Missiles, Rockets, Satellites, you name it.
Before the USSF the closest thing to this was a couple sub-departments within the USAF, US Army, and USN plus the NGA (which is just a supporting agency).
This has been something on the books for decades now [0]
At the end of the cold war. In the present day, a launch is inconceivable.
That's why we don't much care if the missile crews sitting around in underground bunkers are napping, playing xbox, wearing pyjamas, taking LSD and cheating on their proficiency exams.
And the fact there won't be a launch is a great thing - No reasonable person would enjoy it if mutually assured destruction was to occur.
Why have we waited until missiles are fading from relevance, years after the end of the cold war, and only now decided they need their own branch?