Re: "if the planet is not under any known threat"
There is always at least one known threat : planet-killer asteroids from the Oort cloud.
I'm guessing that practically all stars have an Oort cloud. Astronomy is in the process of finding out that apparently most stars have planets. It would be very surprising if those planets did not have asteroid strikes every know and then.
Our specific situation is that Jupiter is shielding us, but that doesn't keep asteroids from slipping past it and being a menace. One day, one of them will not miss us. We need to have a self-sustaining colony somewhere if the human race is to endure.
The aliens, wherever they are, are most likely under the exact same menace. The only problem with this menace is that, as long as you don't have proof that an asteroid is on its way now, you don't have any real sense of urgency to get colonizing.
The problem, of course, is that if you dawdle until the asteroid shows up, it's too late to start a colony and your race is screwed.