Why the Aliens Are Not Here

Here are a few reasons why the aliens aren’t here (yet). This list is by no means complete, so feel free to give your own reasons in the comments as to why the aliens haven't shown up.

• The universe has more aquatic worlds than ones with sizable land masses. We’re surrounded by ocean worlds populated by smart squid that will never leave their world, much less contact ours.
• They’re evolving in an environment that precludes ever leaving, such as any aliens developing under Europa’s icy shell or in the atmosphere of gas giants.
• The aliens drop by every few million years, and the last time they were here, cave people just looked like another species of walking chimps.
• They visited a couple times when the dinosaurs were running around and think the Cretaceous–Paleogene will take another billion years to recover from before it is worth visiting again.
• They sampled our biosphere eons ago and consider our world too toxic; they’ll never make contact for fear they’d spread oxygen spewing bacteria to other worlds.
• The aliens are waiting for our signal and are a million light years away. They’ll hear us – in a million years – if our signals are strong enough to still be heard over the interstellar noise.
• They’re listening for a sure sign of intelligent life, a signal on frequencies overshadowed by natural RF output from Jupiter or the Sun.

• The aliens were next door but died out a million years ago. We’ll never know they even existed unless we find their remains and can understand them. And that’s assuming they’re on a world we could actually visit.
• The aliens have a technological base that precludes space travel, such as a biological industrial base. They could build cities and civilization but not space ships.
• The aliens have too small of a population. They might be incredibly smart, but if they’re as concentrated in a small area as koalas, they’ll never leave their world and are unlikely to develop advanced technology.
• They are close enough to detect our signals but don’t care.
• We’re literally the wrong kind. They only consider hydrogen-breathers, reptiles or carbon dioxide breathers intelligent.
• They’re listening / watching for a type of signal we can’t or won’t ever produce. For example, they’ll only visit when they see our sun start to be surrounded by a Dyson sphere.
• Their biochemistries are so slow that we live in the fast lane. They’re running off liquid helium or methane, and we’re literally too fast to ever make contact with.
• They had technology and collapsed in an apocalyptic disaster; by the time they’re able to listen again, our civilization has collapsed.
• They send probes through planetary systems every so often but never land, much less make contact. Safer for everyone that way, and cheaper, too.
• That weird looking bug you ignored last week? That’s them tracking what we’re doing.
• All the action is around the galactic core. No one comes out to the boonies, the Galactic Rim, and the rare species that evolve here don’t develop interstellar travel because it takes too long to be worth it.
• Aliens did show up, seeded life, and left. That’s all they cared to do.
• There are no other advanced species. The Great Filter is the once in our planet’s history accident that created complex cellular life. The rest of the galaxy has only single celled goo.
• They know we’re here but won’t show up until it is proven necessary to wipe out the competition.
• Their interdimensional portals weren’t recognized by early people beyond a mention in religious literature, and we’ve missed our only opportunity for the next 20,000 years.
• They saw our reality TV shows and decided there is no intelligent life here.