Yes, it makes no sense. It's one of the longest running complaints from the Foundry community. It has frustrated every new author in the history of this tool.
We don't know why we can't, except that maybe it could be used as an exploit for group enemy npcs. You could click a button and all the enemies would disappear.
Why this is also restricted for individual npcs, I don't know. The best explanation that we've gotten came from Zero. It was something like, "We can't let you hide a npc, because you could make a mission that is broken."
I can make a mission that is broken 100 other ways too.
No one understands why. We have to go to great lengths to fake it.
By the way, you can get realtime help with your questions here.
The way I understand it, when you spawn out any object, you "kill it" in the game's terms. So if you tried to spawn out an NPC, he/she would die, fall to the floor and then disappear. Not exactly what most authors look for. I imagine it would also give you XP and drop which would make it even more weird.
Not to mention the possible exploit where you would be able to rack up insane amount of XP by killing high ranking NPC groups with triggers.