I suspect that a lot of this is down to the definition of life that has developed over time. Everyone has a sense of what is, and what isn't, alive. Over time, the application of these definitions has led to some curious beasts. Are viruses alive? In many respects they behave like they are, but then there are odd properties as well, for example it is possible to form crystals of the tobacco mosaic virus.

It is also possible that we could build a machine (with cogs, etc) that would meet all of the definitions of being alive. At the very least, nothing seems to rule this out. I suspect, however, that we would modify the definitions in order to exclude such a beast. (A related example is the "tinker toy" brain. If consciousness was simply the product of a sufficiently complex Turing machine, then it should be possible to build a conscious brain using wooden sticks, wheels, and gears. Even if it fitted our definition, we would find it difficult accepting that it really was conscious.)