They missed two big causes.
#1 Hording: If you've got a house full of crap, and you're deeply ashamed of it, you're not going to be having visitors over, ever. You're also not going to mention it on surveys.
We, the American public, have been collecting shit at a prodigious rate. Many of us pay rent for stuff that won't fit in our home. It's why the storage industry is so reliably profitable.
#2 Paranoia: Otis Toole[1] kidnapped then killed Adam Walsh. His father publicised his kidnapping, and fueled a moral panic that sent American parents deep into paranoia. Kids used to be free range, and were often collectively instructed to "go outside to play, and come inside when the street lights come on". After that event, we're afraid to even let them walk to a friends house a few doors over.
The end of the fairness rule, and the rise of fear based media like Fox news certainly didn't help in the rise of distrust.
[1] https://en.wikipedia.org/wiki/Ottis_Toole#Murder_of_Adam_Wal...
#1 is a big reason for our house. The house (and garage) badly needs a significant declutter, but we are both working professionals and don't have the energy to make a significant enough dent, especially since we both have hobbies where we accumulate more stuff all the time as well (especially me, I impulse buy board games and don't cull the ones I'm not playing often enough).
It doesn't help that I design board games too, and have too much of an easy excuse to want to get these games so I can 'research' them.
And because I'm mentally tired all the time after work, that also seems to lower my willpower. And clicking the 'buy' button online is way too easy to do.
We did used to host a monthly game designer playtest night before the pandemic, but our house had a lot less stuff back then. And being stuck in the house during the pandemic with nothing to do but play solo board games and video games (and also the uncertainty if I was even going to make it through the pandemic, since I already had respiratory issues from asthma) definitely accelerated the bad impulse buying habits I already had.
We have made slight dents in it, but the significant declutter probably won't happen until the next time I'm in between jobs, assuming there's at least a month in between jobs.