Because of evidence. There is none which supports this view. (Note: In case anyone should misunderstand or pretend to misunderstand. I am not here talking about the lack of evidence for life on other planets. I am talking about the lack of evidence to support the claim that a majority of scientists think it likely that there is life on other planets).
Conclusion section of the NRC's Limits of Organic Life in Planetary Systems
From the other end of the spectrum, here's a quote from Ward & Brownlee's
Rare Earth: Why Complex Life is Uncommon in the Universe (italics in original):
"In this book we will argue that not only intelligent life, but even the simplest of animal life, is exceedingly rare in our galaxy and in the Universe. We are not saying that
life is rareonly that
animal life is. We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex lifeanimals and higher plants is likely to be far more rare than is commonly assumed."
There is a wealth of information in astrobiology-based volumes alone (see e.g.,
Origins and Evolution of Life: An Astrobiological Perspective (
Cambridge Astrobiology)
, Cambridge University Press, 2011;
Life in the Universe: Expectations and Constraints (
Advances in Astrobiology and Biogeophysics), Springer, 2008;
Chemical Evolution and the Origin of Life, Springer, 2008) on just how many scientists in diverse fields consider it to be ludicrous to imagine that life doesn't exist elsewhere. Far less clear is how many have good reason to suppose this is true for anything other than the type of life we find in unbelievably extreme conditions on earth. But this:
I am talking about the lack of evidence to support the claim that a majority of scientists think it likely that there is life on other planets).
is simply not true. There is, and has been now for some time, an interdisciplinary field of biologists, physicists, cosmologists, even computer scientists called "astrobiology" and a wealth of literature about the consensus of scientific opinion.
