I think it's because LLMs tend to obsessively write in a this-therefore-that kind of way, where an emdash is often the most semantically fitting punctuation.

Apple devices produce a ton of emdashes; "Smart Punctuation" is enabled by default on iOS and iPadOS, maybe Mac as well - and it's at least a decade old.

I've had to disable it on all my iDevices bc it breaks some Markdown parsers. How big a % of training data was produced on an Apple device? Ehhhhh probably small.