Part of the reason is probably that nearly all high income countries have terrible housing policies that make it impossible for ordinary people to have stable housing until their late 30s/early 40s.
Leaving home early is deeply ingrained in Anglo culture.
1. https://en.wikipedia.org/wiki/Western_European_marriage_patt...
The war on affordable housing is probably going to continue in the west until there is some kind of systemic break (war, societal collapse, rise of a "caesar", etc). There is simply too much wealth riding on it and too much wealth feeding political corruption for any kind of sudden trend reversal.