Why do they ask in interviews if I still live with my parents?

Anyway, in an interview last week, the hiring manager asked me point-blank if I live with my parents. Honestly, it really confused me. Then she started asking me about my commute to work and how I manage my personal expenses. I gave a somewhat vague answer, but she was insistent on knowing.

And the weird thing is, this is the second time this has happened. Another company asked me the same question a few months ago. I ended up rejecting their offer because the whole thing felt uncomfortable, but now I'm starting to wonder if this is a normal thing that happens?

Seriously, are they even allowed to ask these things? I feel like they're very personal questions and I don't see what they have to do with the job at all. I'm a recent graduate and new to the corporate world, so I honestly have no idea what the protocol is in situations like this.