the way they train the models is feeding it lots and lots of literature, and usually high quality.
The way they write by default is thus going to be this weird hybrid of all english styles/dialects from the last ~500 years.
The reason they are heavy with em dashes is that they were immensely popular in literature for a long time but not so much in modern times. So it stands out.
If you tell it to write in a specific way though, it does a good job at it.
Here's detroit english, no messin' around. The reason why AI's love the em dash so much is simple: it’s the most versatile and natural punctuation mark they can use to connect ideas and maintain flow. A large language model's primary goal is to sound human, and when people speak, they often pause, clarify, or insert a quick side-thought the dash captures that conversational stop-and-start rhythm better than a rigid comma or a full-stop period. Plus, in the enormous amount of text the AI studies (its data), the em dash is frequently used by skilled writers as an efficient tool to replace colons, parentheses, or strong commas, so the AI simply picked up that effective writing pattern and runs with it, seeing it as the clearest and most dynamic way to structure complex sentences. That's the real deal.