I think that for the majority, living with parents when you are adult is a huge psychological burden. Not only the society that does not approve such lifestyle, but also the fact that an adult person must have their own personal area to which their parents do not belong (sexual life is one example).
When one is a baby, they need everything taken care of for them. Once you become a toddler, you can probably eat by yourself if someone puts it down in front of you. By the time you're 6 or 8, you can usually do fine in the house all on your own for a few hours at a time, and so on.
The main problem is when you're a teenager, I think, because you think you know everything and your parents can realize all of the stupid crap you're doing, so there's a big clash there, but once you progress through that stage, things are easier again, as long as your parents are willing to give you an appropriate amount of space.
I think moving out _can_ be important and helpful. I just think that our society is too judgmental of some who've chosen that they prefer to be under the same roof as their parents. There's nothing necessarily wrong with that, as long as both parties are sensible regarding it.
I don't think it's universal at all. I grew up in the US with grandparents and uncle and even great-grand parents at home, and it was great.
Here in Taiwan, tons of people live in extended family homes, too. For the most part, it seems good for everyone involved. Grandma and grandpa can help watch the kids until the parents get home, so the parents don't feel so overwhelmed, the grandparents feel useful and get to enjoy family life, and the kids get more support... especially if there's also a younger auntie or uncle (with or without a live-in gf) there, too.
It's a good system.
If your parents are going to treat you like a child and/or you don't have an open enough relationship with them to feel comfortable bring home a girlfriend / having sex with your wife, then there will be inevitable conflicts.