Why does nuclear delivery has to be manned?
It's all about putting the decision of pulling the trigger (or pressing the button) in the hands of humans.
The militaries certainly can train their machines to leave the decision to launch (at least certain types of) conventional weapons to unmanned (or even autonomous) systems in order to improve respinse time and minimize command backlog - But nuclear weapons (which are WMDs by nature) is another story.
Remember how the Russians shot down their own S-70 Okhotnik that went out of control in October last year? They had to shoot down their own drone using an Su-57 because they didn't manage to reestablish connection and control over the drone before it flew past the frontline and into Ukraine.
Imagine a drone is carrying nuclear weapons and tasked with conducting nuclear deterrence/"show-of-force" patrols along the Russia-Ukraine border - Or worse, Russia-NATO borders, and having the same problems as that ill-fated S-70. Even by leaving the risk of accidental nuclear detonation aside - Conventional explosives that could be accidentally set off would result in radiological disaster at the location/area where the drone crashed after running out of fuel, let alone the associated diplomatic disaster that will surely follow (if not a military response).
We already have plenty of nuclear close-calls throughout the entire Cold War. We don't need more.