Except what this site says is that counter-intuitively, this policy doesn't save lives, it's actually the opposite:

"Zeidler explains that “this appears to be a reasonable precaution for life safety. However, the life safety of this single stair type in Europe has been equal to, if not better than ours,” due to the fact that the single stair scheme is shared between fewer units."

And that's the thing, entire countries disagree on the necessity of a second stairwell for fire safety, and there's data to back up the position.

But because a lot of people in North America have those (unfounded) fears, the second stairwell is part of the building code, and as a result an entire class of buildings don't get built because the economics don't work out.