I was a teen in the late 90s, and was friends with a lot of the druggie/stoner/party crowd. By and large, these people do not have kids, because they can't afford to. A good many of them are dead now, either from overdoses, or drug-related accidents (like crashing their car while going 100 mph while high), or suicide.
The 55 years of American history post-WW2 was a bubble, where people could get away with a number of not-very-adaptive behaviors because the whole country was on top of the world. The last 20 years has rapidly exposed this as a bubble, and the folks who were swimming naked when the tide ran out didn't leave offspring to be part of the next generation.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3449224/
If you look at only populations who were likely to go to college in the first place - say, white (formerly) middle-class two-parent households, which is what the bulk of my high-school friends were, there's a positive correlation between educational attainment and fertility. Simpson's Paradox strikes again...
At least in the Bay Area teens appear to be way more buttoned down and smart than my generation. Don't get me started on the boomer generation. My teachers said my and even more my younger sisters generation were a very welcome relief from those guys.
But no, it's because they're rowdy kids doing 'bad things'.
And then there is the other 2/3 of society, who have been going downhill economically for decades.
