> - dash: compounds word-parts (Sarbanes-Oxley)

That's a hyphen, and (to be pedantic) style guides usually do not consider it a dash.

There's also the 2-em dash in some style guides for redacting words, and since care needs to be taken to avoid a line wrap between the em dashes, Unicode has a character for it: ⸺

Finally,

> - em dash: adjoining thought (Hey, so — <declaration>)

This use is sometimes discouraged by style guides, and in many cases, it can be replaced with a colon, comma, or semicolon.

In general, even before AI, the em dash is often considered overused.[0] My guess is that the em dash is perceived as sounding educated, so novice writers who learn about it end up using it recklessly, and LLMs could've picked up this habit from them.

[0]: https://slate.com/human-interest/2011/05/em-dashes-why-write...