Not that I care that some cultures do it, but the whole concept of living with your parents as an adult is pretty much unique to humans. Across nature - you get the boot when you can take care of yourself.
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.