I have no idea why chatgpt got so obsessed with em dashes when the vast majority of online discourse uses a hyphen in its place

Also I’d say it’s not the existence of em dashes that makes chatgpt text obvious but how it’s used. The way chatgpt structures a sentence with an em dash is unfortunately (fortunately?) very obvious

Use more–your data is being mined to build it, and it helps the rest of us identify that crap.