Em-dashes would be a loss, they can be a better flowing version of a parenthetical. “It’s not X it’s Y” is not a loss. This phrase is a symptom of a situation where the author wants to subvert expectations but doesn’t have the space, ability, or faith in their audience to organically set up X as the thing to be contrasted against.

I suspect it became an LLM tell because it is over-represented in text that’s easily available to the models but that most people don’t actually want to consume: marketing text, LinkedIn posts, that sort of thing.