Mummy - I think it's because in this country, we don't normally defer to our parents once adults, which other cultures do, so living under the same roof doesn't seem as odd in those cultures because you wouldn't be expected to be fully independent from your parents even if you were living elsewhere.
In this country, it does seem weird, because it's assumed you'd want independence. I guess I find those who've lived at home for an extended time after leaving education as rather selfish and childish, because they aren't having to face adult decisions, they aren't having to budget properly, they aren't responsible for a home (even if you rent, it's your responsibility to keep that to a standard the landlord wants and responsible for the rent), they often aren't the one deciding what's for dinner and cooking/cleaning (even if they help out), they are usually seem to have wages that are veiwed more as "pocket money" than money to live off.
It's also not deciding your own house rules, but living by someone elses (which follows into the overnight guest rules, it's someone else's decision if you are allowed, and you have to think about your mum hearing if you want to be noisy). You aren't deciding how you will live your life, someone else is, and you are deciding if you can put up with it, you don't decide the rules, just if you can tolerate them. That's very different to being fully in control of your own life.