Because skilled writers have started using in place of semi-colons and parentheses in modern writing - it just feels right!
AI models are trained off the work of these writers, so they use them now. Except that the difference is that a writer will just use the - next to the `0` key, expecting that Word, Google Docs, or whatever is turning their markdown into rendered text will automatically turn it from - to an em dash, which is a different character.
The models are then provided that rendered or autocorrected text, and return text with the em dash, instead of the one we type.