I think that the use of em-dashes specifically is a result of either the preprocessing of the training data or postprocessing of the generated text. I doubt that the material the models are trained on (i.e. Reddit) contains more em-dashes that hyphens in the position of sentence breaks.
But it definitely gets the use of dash as sentence break from people writing like that. If you ask ChatGPT in another language, whose users don't generally use dashes, e.g. Slovak, it won't use then as much.
I think that the use of em-dashes specifically is a result of either the preprocessing of the training data or postprocessing of the generated text. I doubt that the material the models are trained on (i.e. Reddit) contains more em-dashes that hyphens in the position of sentence breaks.
But it definitely gets the use of dash as sentence break from people writing like that. If you ask ChatGPT in another language, whose users don't generally use dashes, e.g. Slovak, it won't use then as much.