Because in her digital home, there are people who will sneak in the back door or find a window they can jimmy open. They'll bring in stolen and illegal stuff, then invite all their fence and dealer friends over to join the party, and pretty soon mom's digital home becomes a meth house.
All of that can happen to your physical home, too. When it does, you call the police. When it happens to your digital home, you track it to some other country, then call their police who will do nothing about it.
Yes, but I do a lot more vetting before I invite someone to my physical home, so situations like these are vanishingly rare. As opposed to a digital home which is free to any nefarious person around the home.
(I do host stuff at home, but I don't think it is good to understate the risks)