Because when I entertain my MIL's worry, it becomes mine.
I'm heading out into a rainy night to drive home. I assume I will be safe. I don't really worry. And when I get home, I'd just go about my business, with no more thought to whether there's some chore left over from the drive, and without thinking, "Wow, I'm lucky to have made it home without mishap."
But now, she is worried, and therefore, I have to drive home holding onto the thought, "I could have an accident, so I'd better call my MIL to let her know I didn't have an accident." She has also given me a chore to remember when I walk in my house.
It destroys my peace of mind.
And, w/ my MIL, I know that if I call when I'm driving home on a snowy night, she'll want me to call on a rainy night, then on a misty night, then on a foggy afternoon. The moment I feed her anxiety, she gets the message that she's entitled to make her anxiety *my* problem to assuage.
It's not a reasonable anxiety. I refuse to make it my problem, to let it mindspace in my head, or to make it a chore I need to remember.
If I were worried myself about the drive, I might not resist. The one time I let her talk me into calling her, it was a bit trickier then normal. But I also found that her request *did* change my confidence level did affect how comfortable I was with the tricky drive. It was harmful to my mental sense of safety. I wouldn't have been UNcareful before, so it didn't really affect my actual safety. Just my peace of mind.