Nothing personal against you, but we see a fair number of self-described Europeans that are a lovely combination of naive, ignorant, and arrogant. It isn't fair to stereotype, but it takes ongoing effort not to.
But are you an expert in it? Have you researched it? Are you sure the world would "work just as well" if the safety requirements were removed in the US? Considering things like different building materials, different rates of wildfires near urban areas, etc.?
Maybe instead of declaring yourself baffled as if it's obviously something dumb or backwards because it's not how you do it in Europe (or another place), you should ask why it's different instead, and see if you can learn rather than judge.
Then it wouldn't come across as condescending.
That aside, I fail to see any cultural or other reason to require twin staircases nowadays other than ‘that’s how we always did it here’. I admit it openly and state that I’m from somewhere where we don’t do that, and the world works just as well, except buildings have a higher useful to total area ratio.