Life appears common, but multicellular life appears to be extremely uncommon and appears to have evolved only once on earth, at least according to Nick Lane's excellent (though inadvertently depressing) book The Vital Question: Energy, Evolution, and the Origins of Complex Life (https://www.amazon.com/Vital-Question-Evolution-Origins-Comp...). I can't gauge the accuracy of his claims but the book does not appear to have been rebutted, at least from what I've found. Given all the discussion about biology on this thread I'm surprised no one else has mentioned it.
The common explanation is that once life evolved and began to spread, that it would prevent other life from appearing by out-competing it. But there is plenty of raw materials and sunlight to go around, so I don't buy that explanation.
The only thing that seems to fit, is that life is hard to evolve, but has extra-terrestrial origin. If that is the case then life should exist (at least in bacterial form) wherever a suitable habitat is found.