Few explanations:
1. They are silently observing us to find something about their own history when their civilisation was primitive like us.
2. They are silently observing us and waiting for us to be mature enough so that we can be admitted into Galactic federation
3. They are just curious and visit us like humans visit bird sanctuaries to watch birds.
4. They are our creators and just observing us like humans observe artificially created ant colonies.
5. They are ethical & patient conquerors and are waiting for humans to end themselves so that they can take over the planet for themselves.
6. They are humans from our own future and observing their idiot ancestors destroy the planet or trying to set somethings in motion in order to change the course of history.