There are billions of galaxies, there are billions of planets in a galaxy, there are billions and billions of stars in a galaxy. There are more than one universe( Who ever bleieves in multiverse)

With this information, I can say one thing that is a theory, because we havnt proved it yet, but there IS life out of earth, another word for anything that is alive out of earth is an extratrerrestrial, an alien.

Now keep in mind i dont mean the little green guys with flying saucers. there IS life outside Earth.

Now your question, why havent we had visitors? Depends, if they want to visit us they can, if they dont want to, they dont have to, simple as that, and we dont know if there is any life within 50 light years of planets, that we know of. So that would take a long time to get to us if they wanted to make contact.

Well, I do know something of value, and that is without outstanding human beings you or I would not be looking at, or contemplating on the pictures you have shown and as far as that goes, human beings are important, we can't even say with conviction what the limit of our 'evolution' can be, only of the things we can't do at this time.