I’ll try to find my Dragon Con presentation from 2014, but here is the question and short answer, for those interested.
Fermi Paradox: With all the stars in the galaxy, there is a high probability of aliens. Why can’t we see them and why don’t they visit?
The first part is simple. We can’t see much of the galaxy. Most of our view is out of the galaxy. If you want to think of our galaxy as rotating around a Z axis,+/- Z is our long view. It is only a few tens of light years. (Still a lot of stars) If you look toward the other directions, we are limited to around a thousand light years.
How big is 100 light years? Imagine that the galaxy is a queen sized bed. Scale runs around 600 light years per inch.
Try an experiment: Take a bunch of quarters and toss them randomly all over the bed. Each quarter is an active, visible, alien civilization. Nice, big pile near the center, maybe a stack by the headboard. A lot of those quarters are touching, right? Those civilizations can see each other.
Good, now put a quarter one foot toward the center from the lower left corner. Did you throw another quarter there? Well, apparently God didn’t either. The Fermi Paradox. There may be intelligent aliens out there, we just can’t see them.
As to why they don’t visit? Because space travel is hard. Star Wars is awesome, Star Trek is sufficient, but both break basic rules. Frankly, if those rules could be broken, they would have been. I expect that we’re always to be limited by light speed. Which basically means our top velocity is 0.1 C. We might plant a colony at 14 light years, but it is a several hundred year trip. The only aliens we’ll ever see with share our DNA.