Actually some of the theories I have seen explain this pretty well. We see the type of life that the environment here was capable of developing and sustaining. It is possible that life developed under far different circumstances and exists in environments we could not. So partly the anthropic principle again, that life developed here because this was an environment conducive to carbon-based life to develop. That isn't to say that some other type of life couldn't have developed in far different circumstances with far different properties, say with ammonia as a base medium instead of water on a planet cold enough to sustain liquid ammonia, but perhaps heated from within. It might even still be carbon-based, but without the water component, and it could represent a far different biosphere than we would think possible. I also am just not a fan of making blanket statements based on our knowledge of physics and science when it is obviously not really that far developed and saying definitively that something could or could not exist. It is fair to say that as far as we understand the laws of physics we cannot see a way that it could work. It does not preclude the possibility that we don't know everything and maybe it can, in a way that would teach us more about the science.
Entire planets of piss drinkers eh?
Isn't it possible to find new elements and new forms of existence as far as you sail away? Even considering the within the Earth's boundaries, we still come across amazing creatures that we could not even comprehend that they existed before the deeper we search the oceans. No elemental critical difference, just a great deal of difference in the conditions, pressure, heat etc. God knows (or Science knows if you will) what we would contact if we successed at reaching Andromeda in one piece and undamaged conscience before we crash.
All these new creatures are made up of the same stuff though.