War of the Worlds: Goliath

There's no good material reason for an interstellar species to conquer Earth, since anything they might want would be more easily mined from asteroids and smaller moons. So that rationale in invasion movies has always made me groan. However, cultural/religious reasons to conquer other planets are perfectly valid. Like the Predators visiting other planets to hunt the dominant life forms. If alien culture decrees that they must prove themselves against all other species, well, let the invasion fun begin.

With 100 billion stars in our galaxy and 100 billion galaxies, there are almost certainly other civilizations out there. Recently some Yale astronomers announced that there may be three times the number of stars previously thought extant since we've not been able to detect most red dwarfs before. (And just when they cancel the show. Go figure.) But those potential civilizations are incredibly far away. Without some way around light speed, they are impossibly far away. Traveling one million or ten million or a hundred million years to get anywhere is a ridiculous endeavor.

Which also brings up the issue of time. Suppose millions of alien civilizations are out there. The odds are that they are billions of years dead. Even missing an alien civilization by a few thousand years could easily mean they're extinct. Or they've "ascended" into pure energy beings or uploaded themselves into machines and are no longer interested in the outside universe.

That's one reason David Brin's Uplift series is so interesting: advanced civilizations tamper with the genes of semi-sentient species to make them fully sentient (uplift), then mentor them as they master their intellect and the resulting society that comes from it. Eventually you have a species ready for interstellar travel, ready to join the galactic book club. Which means civilizations are all pretty much even-steven. Same sort of thing in Larry Niven's Known Space books, where billions of years ago there was a galaxy-wide war that drove every sentient creature extinct. As a result, new civilizations all got their start at more or less the same time from the dregs of those earlier ones.