Why? I lived at home until I met a man I wanted to move in with. It's not that unusual for women to do that. I didn't see the point waisting money on rent and didn't particularly want to live with a house mate.
Yes she has a well paid job and could afford to live alone, but it is expensive living alone and can be lonely, so why not stay with mum and dad. For all we know she's got thousands in the bank just waiting to buy a property when she meets the right person.
I don't know why it is so looked down on to live with your parents in this country. In other cultures it's perfectly normal. Not everyone hates their family and can't bare to see them more than a few times a year.