It all falls into one overarching category: style over substance, quantity over quality. Em-dashes are a simple way to sound important, same for generic throat-clearing phrases like “It’s important to note that…” plus they puff up the text without saying anything, same for overly symmetrical structure like neat triplets parallel clauses, and balanced pros/cons even when the topic does not naturally call for them, etc.

And they don't have a sense of stylistic restraint, so they often go overboard with one or more of the above.