They are some of the first Settlers on this planet, this strange planet that is so different from the other's they have visited.
This planet seems far more aware of the threat of alien 'invasion', even if people who believe that it is possible are scorned. There are some people that will be willing to fight back, and maybe even win.
Yes, Earth is very different to other planets they've been to.
But they've been prepared for these kind of eventualities, to keep quiet and small, moving slowly. They don't reveal themselves until they are sure that they won't have a real fight.
The invasion is slow and quiet, and no one knows it's happening until it's over.