My suspicion is that the causation mostly goes the other way—LLMs write like that for the same reason that many humans do, namely, that it's a cheap trick for sounding smart with limited effort and cognitive capacity. (My guess would be that em-dash usage among human writers is down in the LLM era because people don't want to be accused of being LLMs, though I don't have any data on this.)

Coincidentally I just read a blog post today that explained this in a way I always struggled to: https://www.astralcodexten.com/p/nostalgebraists-hydrogen-ju...