The point is that we have a relatively small sample of the conditions under which life can arise. Even in the extremes on Earth, we have found life. Often, it is less complex than life in what we consider to less extreme environments, but we also have to consider that 'extreme' is relative. We can point to factors that would make it inhospitable for most complex life on Earth. However, it's quite possible that Earth has equally strong factors against it that would make the majority of sentient species surprised that Earth can support life. Cursory research says that the biosphere is a mere 0.0007% of the Earth's volume, and there may be a set of conditions that can support some kind of advanced life in a different minuscule percentage of this planet's volume.