I've always wondered about this too, but kind of landed on something like this:

Sure, there might be (and probably is) life out there that falls so far outside our definition of "life" that we would never detect it. If it's undetectable, and maybe even incomprehensible, to us, then what's the point in even thinking about it in the context of a "search for life"? What we really mean by the question "Is there life out there?" is "Is there life out there that's similar enough to the life we see on Earth that we could recognize it as such and interact with it?"

I don't have a good way to phrase what I'm getting at without sounding dismissive - I do think it's interesting to think about other forms of "life", but it seems almost philosophical at that point and not scientific.