I'm guessing it's because it's a level of familiarity thing. When I talk about my husband, I might use my in front of his name, or say my husband, Dan, or even Dan Smith if there are several Dans and I want to be clear. I don't want to say Dan and just assume everyone knows I mean MY Dan. When someone says, "Mom is in the hospital" it's kind of like Mom is her name and not acknowledging the fact that most of us use the name Mom to refer to our own mothers, so it at feels either overly familiar or kind of proprietary. Although it's not something I've really noticed or that bothers me. If I know the person well, it seems fine.
My church choir director always refers to her mom as Mom, and I know who she means now, but when I was new to the choir and she said that Mom was sick, I didn't actually know who she meant because I didn't know one of the women in her choir was her mother. But I just am realizing now that she always says Mom to us like it's her mother's name, and it doesn't bother me, although I call her mom something else. I wonder, though, if it's regional or if it's simpler because we all know her mother. But then sometimes people whose moms I've never met do the same thing, so I don't kno.
this - the first part. And it's not a huge deal, but we are discussing pet peeves, so.