One more possible solution.
They have seen the earth and how we behave and have no desire to visit here.
I think the vast distances involved would preclude travel even by robots.
The closest star is several light-years away, and not every star has planets. Of those stars which DO have a planet system, very few of them may possess a system capable of sustaining earth. Not only would a life-sustaining planet have to be the proper distance from its star, it would also have to have a moon like ours. The moon provides the earth with some kind of astrophysically essential ingredient for the existence of life which I can't recall.
Also, we are not sure that evolution on those few planets that may be capable of sustaining life progressed to the level of intellignet life. Intelligent life as we define it, may be a rare or non-existant trait in other worlds.
variations in tides.
>>They have seen the earth and how we behave and have no desire to visit here.<<
I've heard it proposed that we would likely be quarrantined. We are approaching the age of near-light speed travel, which would create the danger of photonic wakes: dangerous for unseen bystanders. But I think the person who brought this up was full of BS...