What!? Kids in earlier times, even as recently as the 80s grew up in a time where they were constantly facing the very real threat of nuclear annihilation at moment's notice! With nuclear drills only being ended sometime in the 90s. In the 80s the we had a political era that included real assassination attempts on the president, the pope, and more, violent crime was at record highs, and more. Today is more "stressful, distracting, uncertain, unstable, etc" how exactly?
And you're again doing what I think is actually quite dangerous, yet also standard practice in the social sciences. While I think your presumption is wrong, you're assuming that not only is it true but if it were true than it would naturally have a causal explanation for another factor. Yet I think there is an extremely strong argument that your presumption would actually have a causal effect in the exact opposite method of which you imply.
Humanity excels when dealing with crisis. Nearly all of our evolutionary existence has been living in times of great adversity. I think the fact we live in such relative comfort and safety now a days is something may be interacting negatively with our evolutionary wiring (such a hypothesis would even explain things such as 'outrage culture' and internet activism - in lieu of problems, create them!) As I mentioned IQs are decreasing (and literally decreasing - not 'increasing more slowly' towards an asymptotic zero), but this is something that's only happening in the developed world. In the developing world, where lives are still much harder and threats more imminent - IQs continue to increase, academic performance is increasing, and so on. Vietnam for instance, today is ranked 8th in the world in science - as they teach with a per student budget a tiny fraction of ours, even parity adjusted. There are clearly many factors to progress that we seem to just have 0 interest in considering.