You are trying to make the conversation about something it is not, and have changed their point to my point (from primary to important but not primary) to then say I am being unreasonable because I don't understand their point, which is in fact mine. There is no reason I should accept to be shut into your box. Read the twitter thread, and this comment thread again, and it is clear you are the one trying to force the conversation to be about one very specific thing while refusing that it is legitimate to place it into a larger context - the overall causes of lower US life expectancy.
The only reason I care is I don't want a bunch of people running around with the false idea in their head that we just need to deal with the important social issues of our younger generations, and we have solved the bulk of the problem. They are certainly important, even critical issues, but that is not true.
I think you are confused about this because you also brought up child mortality. Yes, child mortality is also much higher in the US than other developed countries. But we knew that. The article is focusing on less discussed yet very impactful factors which disproportionally impact young people in the US.