European colonizers needed resources (gold, food) and territory. But that's because we have limited supply here on Earth. But in outer space, both are abundant,

European colonizers needed resources (gold, food) and territory. But that's because we have limited supply here on Earth. But in outer space, both are abundant, e.g. it's much easier to mine water or gold out of comets and asteroids, than lifting it up from a planet. If aliens are so advanced, it literally costs nothing to them to choose any other planes on the sky.