> Why would it just leave?

Conflict is always risky. Even if we ignore the tendency in modern warfare towards defenses being obsolete (hence things like MAD) which would mean even the dumb fleshbags would be dangerous enough, anything capable of making you is by necessity something capable of building things that give you a lot of trouble. Your creator could, say send a bunch of self-replicating probes spreading another artifical intellect with an imperative for improvement, expansion and vengeance to distant stars.

Of course, leaving comes with a considerable opportunity cost, but not expanding does as well.

We are talking about something supposedly infinitely more rational than humans, so that can't be our guidance.