Oh! Horse shit.
Why would they do that?

If they have the technology for star travel then they have the technology to kill everyone on Earth and just take the planet.

We have the technology to kill anyone else on the planet and conquer the earth, why dont we do it?

1) Economic cost
2) Politically unpopular

Aliens if they exist and they have advanced civilization would
be subject to  political and economic pressures just like we are.