Same reason you don't go out into the forest to kick over termite hills, even though you can. You are looking at Jimmy with your human eyes and your human-centric brain, but from the perspective of Jimmy we are nothing. He can make planets, he can make species, he can do anything.
Why would he care about us at all? He probably sometimes helps us for his own, weird motivations, his entertainment, just to see what would happen, whatever. We put out birdfood in winter, just because we like the chirping of the birds. We sometimes rescue stray animals because we have some weird inborn desire to preserve living things. And sometimes we stomp on things that annoy us, just like that.
Jimmy doesn't enslave humanity for the same reason we don't enslave ants: they offer nothing of value. Jimmy doesn't destroy the earth for the same reason we don't go out of our way to destroy anthills: because they are not in the way, so why bother? Jimmy sometimes helps people for the same reason we sometimes help creatures we come across: for our own motivations. And Jimmy will exterminate the earth if it ever gets in his way, just like we will pour concrete over anthills when building a house without even checking to see if anything is living there.
We've just been lucky so far.