Why would an advanced civilization that could travel light years to reach us fear us and hide from us? Stupid, stupid, stupid. When Cortez hit Central America, they all hid in the woods so the Inca’s and Aztecs would not see them. Columbus did the same, they hid their boats and dressed up like trees so the native americans would not see them.
The advanced civilization is visiting a fallen world where Satan is kept and will be released for a little while. IMHO
To avoid violent encounters that might result in contamination.
Because if they revealed themselves, they would likely become involved in our politics and issues. Imagine the social unrest among those whose ideology and ethos cannot include the existence of extra-terrestrials. The middle east cannot even handle cartoons. Imagine a spaceship landing.
Do you make yourself known to the insects and creatures of the forest when you go hunting or gathering firewood? They likely see us the same as we see lower-life forms and our existence and our affairs are likely beneath them.