Em-dashes historically been very common in nonfiction and literature and are a very versatile way to compose more complex sentences and provide an alternative to comma-separated clauses, colons, parentheses, etc. I think AI is just reflecting something that fell out of common usage as part of a general trend of people reading less and paying less attention to punctuation and sentence construction. Now it's seen as a tell for someone using AI, but if you pick up any book written in the last 200 years you'll see tons of em-dashes.