I'm gonna take a guess and say they realised humanity was destroying itself with far more efficiency than they could, so they saved themselves the energy.
Why AI should be so dumb to openly show himself and start a war with us when it could easily take control over our media and economy and push us into self-extinction?
I recently saw people posting about the "voluntary self extinction" movement the other day, which aims to lead to human extinction. And people are genuinely buying into it.
If that's not some rogue AI psy op, I don't know what is.