Why?
Oh FFS. Do you really not know?
This, just like so many other things in our current society, is simply a direct result of the fact that over the last few decades, the system as a whole has shifted sway from the well-being of the many and toward the privilege of the wealthy and powerful few.
Put bluntly, younger people aren't partying (or having children, or buying homes, or planning careers, or supporting establishment political parties, or even going outside much) because the earlier generations have turned the US into a toxic shithole run by and for lying, greedy assholes, so the younger people see no reason to plan or hope and no opportunity to even really look up from their day-to-day gig economy scramble just to fucking survive and the handful of things they can afford to do to wind down from that scramble.
You want to do something about it? Stop raining shit down on them. Tax the fucking billionaires, shut down the student loan opportunists and the health insurance opportunists and the housing opportunists and all the other pieces of shit enjoying government-enabled privilege on the backs of the common people, and do what you can to turn the US back into a place where ordinary people vould have some hope, instead of just facing a life of underpaid drudgery just so that some fucking billionaire who owns a stable of politicians can buy more mansions and a bigger yacht.
And if you can't be bothered to di that, then you're just going to have to accept the fact that the people on whom you're raining down shit are increasingly hopeless, and that has consequences.