The first theory is that all aliens sleep:
Do not laugh. It’s not about the natural sleep that we experience every night. This theory assumes that aliens are beings that are very advanced and have managed to put their bodies in a state of suspended animation, a kind of stasis.
The second theory would be that aliens might hide from us:
There are many reasons why aliens may hide from us. One reason could be that they have created humanity as an experiment and want to remotely observe without being aware of their presence.
Another theory might be that aliens are still evolving:
We may be the most advanced race in the universe, and even if there were other races, they could be at the beginning of evolution and would not yet be able to travel through the universe.
Also, maybe we humans have not spent so much time looking for these extraterrestrial races. Today’s people focus on other things that they consider more important than the discovery of aliens.
The most interesting theory is that aliens are already on the ground.
Maybe they are among us but we have not yet realized that. A very advanced civilization could easily hide in oceans or underground bases. They could walk the street among us without seeing them. It could even have camouflage technology.