Because ChatGPT uses it in almost every single response, and most humans do not. It uses the full em dash (—) whereas humans usually use the en dash (-), becau

Because ChatGPT uses it in almost every single response, and most humans do not. It uses the full em dash (—) whereas humans usually use the en dash (-), because it's actually on your keyboard. It's as simple as that. It doesn't prove something was written by ChatGPT, but it's a useful heuristic.