Yes Paula, you are absolutely right that Aliens have been interacting with us in plain sight, for centuries...Here is proof of them communicating with their mother ship, and informing their species that Dogs have human masters, but Cats have human Servants, FACT!! ..
While I believe there may be many reasons, I'm personally under the impression that we have been "visited" for many years. Now why haven't *we*, the public heard about it? That's another story.
Personally, I think that the government has equipment that's been salvaged and just don't want anyone knowing about it. I also feel that the government is afraid. They're afraid to tell us, "Look guys, there are aliens coming and going but there's nothing we can do about it. They can do to us as they please and we can't stop them".
Earth has been visited. Likely many times by small groups of travelers.
The reason that Earth hasn't been colonized by another race is the fact that sending a starship with enough individuals to overpower Earth would be a massive endeavor. Sure, movies show city-sized ships moving through space all the time, but to actually get one of those things to move at light speed without smashing into any number of things would be quite the feat.