Aliens Not In The Same Elemental Plane
We always presuppose that aliens are somehow humanoid or similar to fleshy creatures, but suppose they are in a gaseous or bio-luminescent sphere that we cannot register nor can they register us? If so, that will explain why Directly Meeting With Aliens hasn't happened yet. Even machines, in a far advanced stage may not know how to 'communicate' with a bio species just like we do not know how to communicate with an amoeba.
I first thought about this when they found that orgasim in an acid lake and realized that not everything that lives requires the same elements. Who knows what's out there that can think and function but we aren't able to recognize it!
Edited: Raving on 4th Jun, 2015 - 10:04pm
I think this would more likely to be the case given how unique our plants biosphere is. I also read a theory aliens are just us coming back from the future. I look at aquatic life and how it has adapted to the most extreme environments and wonder if a being could evolve that does not breath air.
I would speculate that it is just a matter of time before scientists discover an organismic that lives on a planet such as Mars.
The odds of being the only perfect place for life to have developed in all of the universe is astronomical.