This is akin to saying a box definitely doesn't have a baseball inside it because we haven't seen what's inside the box. Again, it assumes a premise to be true because it hasn't been proven false - this is a logical fallacy.
Just because we haven't observed such a planet does not mean it doesn't exist within our sphere of the observable universe or that we should assume it isn't there.I'll be as clear as I can:
As far as our telescopes can see, and as far as we can measure the levels of hydrogen, carbon, and oxygen on other planets and their distance from stars, there has not yet been an inhabitable planet as far as we can observe.
Thus far, we have only observed 353 extrasolar planets ; it is ridiculous to assume that within an observable sphere of 78 billion light years  that no life-supporting planets exist. Again, you are arguing from ignorance and using a gap in evidence to support your premise - this is a logical fallacy.
Yes it does. It matters a great deal.My original point to the OP stands, that is doesn't really matter if there are "aliens" out there somewhere in the universe.
Assumption sans evidence. We do not know one way or the other.The distance for travel to or from is virtually insurmountable...
Making an assumption without evidence is never the best option....for all purposes related to us in the next several hundred years, as a species it's best to assume we're alone on this tiny blue speck.
 - Planet Quest: New Worlds Atlas
 - BBC NEWS | Science/Nature | Astronomers size up the Universe