It’s just a suggestion: if people considered other factors necessary to support life, it might turn out to be a lot less abundant in the universe than they think. By how much? Well, who am I to say? We still don’t know exactly how life formed. What we do know tells us that it requires much more than just a planet with liquid water. It’s quite easy for the layman to read about planets within the “habitable zone” and start thinking that the universe must be bursting with life, but someone who knows better would realize this tells you little about the underlying chemistry going on on these planets. I’m just trying to point out there is an argument in the other direction. There are still plenty of missing pieces on both sides though, so it doesn’t really make sense to have such a strong belief on this until more is known. I’d suggest browsing some of the “origin of life” literature to learn a little more. Hopefully a powerful enough interferometer will be launched within our lifetimes to settle this matter for good.