Emergencies are inherently local. Even if smartphones had been widespread in 2001 and the presidential alert system had been active on 9/11, what purpose would a nationwide alert have served?
[1]https://en.wikipedia.org/wiki/2018_Hawaii_false_missile_aler...
As absolutely mind-boggling as nukes are, I think people today credit them with more destructive force than they really have. For one thing, blast force goes down with the inverse square of of the distance. There's very much a diminishing return to bigger nukes, and the area of total destruction will always be much smaller than the area of sorta-destruction.
So if you have a 15 minute warning, you can get away from windows or perhaps go to a basement or subway, There are plenty of ways to drastically increase your odds of survival if you are in the middle-zone of a nuclear strike.
Any all out nuclear war is going to have a lot of people die. The purpose of national warnings and preparedness is to just make that number be smaller than it otherwise would be.
The local FEMA alert system is a good idea; the last time I personally found it useful was in 1989 (Loma Prieta earthquake) though I hear it works in more dangerous areas (e.g. tornado alley) and am not surprised by that. However here in the Bay Area its link to phones was ruined by spam (pointless interruptions sent to my phone) so I disabled it. This new system is worse.
(BTW I wouldn't classify the false missile alert in Hawaii "spam". It was simply an error. But here I got messages that should have been on the news).
The idea is that it would only be used in the most extreme of scenarios that affect the country as a whole.
There’s your answer.