I would explain it this way (of course I am generalizing & stereotyping a bit) - it has nothing to do with "families living together",
"extended families" etc. American WASP culture has always had these things & they are not necessarily viewed in a "negative light" as you suggest.
However, American WASP culture highly values & assigns status to self-reliant "provider/producer" men. A man is not a man to be respected if he can't take care of (house, feed, clothe, educate) himself, his wife, & his children.
Children (esp. men) over the age of 30 living with their parents out of economic necessity are not considered to be self-reliant provider/producers. Inability or unwillingness to Provide/Produce, for whatever reason, is low-status and not worthy of respect.
This is how I was brought up & a view I still carry with me (& I'm not that old). However, I think WASP society is changing in many places & these values are not quite as prevalent as they once were. I do think these values still exist in much of "flyover" America though.