I think it's MOSTLY a cultural thing or a finances thing, but I think there are more and more people for whom it IS a spoiled thing. I think it's a big parenting trend for parents not to want to see their kids struggle. They'll let them stay home until they're ready to buy a house the equivalent of what they've grown up in or better and can afford to fully furnish it.
I lived in plenty of shared apartments (four girls in a two bedroom, one bath), dingy smelly apartments, and unfurnished apartments I couldn't afford to furnish. I slept on a palate on the floor in an empty apartment for a year when I first lived by myself. Almost no one I knew thought it was strange at all. We were all doing variations on the same theme. Most people I knew also worked a "second job" at some point when they were first starting out.
Now there are tons of people who would never dream of sharing a bathroom with a roommate, living someplace without all the amenities, not being able to "fix up" their place beyond dorm room chic, etc.
I WANT my kids to experience the fun of just starting out from nothing. I'm serious about the word fun - it WAS fun.
I think you are really on to something here.
So many people in this thread have said they "cannot afford" a place of their own and would not have roomates.
First off, if I could not afford a place on my own I got roomates. I am an only child--I didn't like having a roomamte, I like my space and am accustomed to privacy (well I was before I had children anyway

) You may not want roomates but it what you do. It is possible to find decent, safe people to live with--you do not have to pick random strangers off of Craig's List. It would never have occurred to me to say if I had to have a roomate I would not leave home

Secondly, the whole "cannot afford safe housing in the town I live in on my salary" thing. If you cannot get a job good enough to cover rent on very basic, small, maybe not pretty, but reasonably safe housing in your area then maybe you need to move. Really. Even a minimum wage job will cover rent and utilities and basic food, etc. (but not going out to movies or eating meat every night, etc.) in MANY places in the USA. We have moved all over the country (and now out of it) going where the economics make sense for us. I can see not moving if you have a family with children in school, or elderly parents who truly need your help, etc but otherwise if that is what you ahev to do to grow up and be independant thenit is what you do.
I HAVE seen many, many people who still live at home because they "can't afford" to move out who have new vehicles, go out to dinner often, etc. Many of them could afford rent on one bedrooms in the town they live in for what they are paying on car payments and entertainment alone. They just don't want to give those things up and struggle with no cable, a tiny place, not going out, no furniture, an old car (or walking, or public transport if possible) etc.
I admit I didn't live alone for more than a few months before DH and I were married. We were so poor (but had moved from Colorado where the cost of living was higher than the possible incomes for us) to Arizona to make it work and we sturggled (are lots of pasta, only did laundry after 9 when the water rates went down, etc.) but we DID have fun (like the pp said) and it made us strong and bound us together AND we can be proud of what we have accomplished on our own now that we are older. Besides, you really appreciate things more when you have HAD to make it work without them
These days everyone seems to want the instant gratification of "the good life" right off the bat without having to work up to it.