Physicist Enrico Fermi postulated that once a civilization attained even rudimentary space flight, it should be able to reach any point in the galaxy in only five million years. The Milky Way, however, is billions of years old.
"So," asked Fermi, "where is everybody?"
One original answer I have seen is that all the alien races we assume are out there are too busy overeating fast food and watching electronic porn to journey millions of light years through space for no good reason.
Now a more technical reason may be in the offing. Turns out that interstellar space flight probably will just plain kill you:
If you get radiation-induced Alzheimer's just going to Mars, it surely means death going to an enormously-farther star.
