Because it’s not really a thing. Here’s Ferris Jabr at Scientific American:
No one has ever managed to compile a set of physical properties that unites all living things and excludes everything we label inanimate. There are always exceptions. Most people do not consider crystals to be alive, for example, yet they are highly organized and they grow. Fire, too, consumes energy and gets bigger. In contrast, bacteria, tardigrades and even some crustaceans can enter long periods of dormancy during which they are not growing, metabolizing or changing at all, yet are not technically dead. How do we categorize a single leaf that has fallen from a tree? Most people would agree that, when attached to a tree, a leaf is alive: its many cells work tirelessly to turn sunlight, carbon dioxide and water into food, among other duties. When a leaf detaches from a tree, its cells do not instantly cease their activities. Does it die on the way to the ground; or when it hits the ground; or when all its individual cells finally expire? If you pluck a leaf from a plant and keep its cells nourished and happy inside a lab, is that life?
This can be a hard thing to accept, because I think that even we evil, heartless, scientism-promoting skepto-atheists for the most part still conceptualize life as a kind of force, a “thing” in the sense that it somehow powers these clumps of organic material and bags of meat that would otherwise lie inert. And when one conceptualizes it that way, it does become rather binary – either it’s there or it’s not. You can’t be “sort of alive” or as Miracle Max might have it, “mostly dead,” in this frame of thinking.
Let’s be honest. Thinking about life-as-a-thing, a manifest élan vital, rather than as a loose umbrella term for myriad biological functions, is to conceive of it kind of, well, spiritually. It’s essentially the soul, which is of course entirely fictional.
To let that go, to stop thinking of life as “stuff,” we (necessarily) complicate the search for it elsewhere. It’ll be one thing if one day we’re visited by extraterrestrials who have bodies and limbs and some means of reproduction. Yeah, those will be pretty clearly alive. But what if some future probe comes upon, say, a combination of minerals that seems to be behaving in a way that recalls cellular duplication? Or we build machines that reproduce themselves but with minor variations to improve (or detract from) their usefulness with each generation? And, you know, what about fire?
Seems to me that we’re best to take each example on its own terms, and accept that, in granular terms, the question as to whether something is “alive” is perhaps too subjective to be useful. That it reveals a bias for our own means of existence and animation, and betrays a tendency for even the most secular among us to fall back on concepts that make sense only in fairy tales and books of myth.