Because like others who do this she Can’t Understand Normal Thinking.
If she were doing other obvious traffic violations would you have spoke up? Had she been driving at an extremely high rate of speed or texting would you have spoken up? You can gently say something without insulting her or starting an argument.
I disagree with those that said it’s not your problem.
You knew what she was doing and said nothing about it. You share in the blame for the frustration other drivers were experiencing.