IMO it's because they're more dependent on their parents than their forebears. And recall how suburban and car-dependent the American Dream is (and really the other Anglo countries; contrast, say, continental Europe), so young Jimmy's not going nowhere unless Mommy can take him there. Could also be because there are more 2nd generation immigrants among these birth years (I'm one!), whose parents would be more likely to be strict and/or traditional (and more of these being successful enough to live in suburbs rather than the ghetto). So they end up just doing school in high school rather than partying or dabbling in drugs and alcohol etc.
And this would carry on even to college / adulthood as more young people attend community colleges, or commuter schools (e.g. Stony Brook, Rutgers, TCNJ, George Mason, the CSUs, maybe even UNC or Michigan if you grew up in those metro areas; basically any 4 year university in a suburb), and live with their parents, owing to financial / job market constraints.
Maybe also autism -> taking rules, laws, and guidance (and education about these) more seriously, and you might as well ask why Gen Z was so good at obeying (and promoting) COVID restrictions, but I'm not entirely sure about correlation and causation.