But it's also often a shoehorned artificial contrast that doesn't really make sense. The Y is often not such a different thing from the X that would make it worthy an actual "not just X but Y" claim. Or the Y is a vague subjective term, or some kind of fancy-word-dropping. It's strong styling but little content, similar to politician CYA talk. I don't think it's necessarily a tech limitation, more of an effect of deliberate post-training to be middle-of-the-road nonoffensive and nonopinionated.
That is possibly one of my personal writing weaknesses that lead my own writing to get flagged as AI.
I can admit "it's not just x, it's y" is mediocre writing - but it's also something mediocre writers do - it's how AI learned to do it!