The simple answer is that if they are different enough from all of the life that we know about that they don’t need those things then it’s likely that we won’t know what to look for to find them. At the distances we are dealing with we can just look for things moving around. We look for planet-wide traits since we can see those, and only know what those traits might be based on our sample size of one living planet.
As a simpler example, say we were using just a telescope to look for trees on Mars. We can’t see a tree from here, but all the trees we know about form big green forests, so we just look for green. There might be other color trees on Mars, but we have no idea which other color they would be, so looking for other colors won’t tell us anything.
Ballatik t1_ixzukbp wrote
Reply to eli5: Why is it when scientists are looking for planets with possible alien species they only look at planets that could sustain human life? by Jcole_Stan
The simple answer is that if they are different enough from all of the life that we know about that they don’t need those things then it’s likely that we won’t know what to look for to find them. At the distances we are dealing with we can just look for things moving around. We look for planet-wide traits since we can see those, and only know what those traits might be based on our sample size of one living planet.
As a simpler example, say we were using just a telescope to look for trees on Mars. We can’t see a tree from here, but all the trees we know about form big green forests, so we just look for green. There might be other color trees on Mars, but we have no idea which other color they would be, so looking for other colors won’t tell us anything.