1) Having your children around after they are grown up is a threat to the health of your marriage and often your own sanity and financial well-being.
2) "Unwanted competition for resources" also applies where the children take advantage of the parents limited resources without contributing their fair share.
Not saying it can't work for some cultures, but in the US and other countries, it hasn't. For the most part, parents simply don't want their children living with them and it significantly limits the available pool of potential spouses/significant others/mates/etc.
In the US, it's synonymous with momma's boys, socially awkward males and those with financially unstable situations.
Let's do it the other way. I'll name only one where living with your parents is considered not kosher - the USA. In practically every traditional society, living in the family home is just fine, especially for those members of the family that don't have their own homes.
In fact if the family home is sizable, raising your own family inside the family home is OK too. I've seen it everywhere I've traveled - much of Africa and South America.
Sure, is OK, but is not ideal, and is done mostly by necessity, for economic reasons.
This would be the case for pretty much any society where land and housing was not easily available. As housing is becoming increasingly unavailable in the United States, and population is still increasing, expect to see it here.
That housing may as well be on the moon for all the good it does.
1. There's often cheaper property within manageable commuting distance from those expensive areas.
2. Those able to work from home are unaffected
3. If you can afford lunar real estate, money's probably the least of your concerns ;)
2. Yeah, and neither are rich people. The well-connected in the USSR weren't affected by shortages of apartments and automobiles, either.
3. On the Moon, I think possession is nine tenths of the law.
It is normal until you either get married, move due to university/job or move with your partner. Why else would you want to live alone? (;
yes, they might been more traditional even in better times, but let's not lie to ourselves that most of the children would not move out if they could afford it