[JohnBooty]

    But do we really know enough to assume that's only way?

I've never heard anybody in the sciences make that assumption. It's just that other biochemistries are speculative at best. Searching for this stuff is time- and resource-intensive, so naturally we look for the sort of life in which we have some expertise.

And to be fair, we are not exclusively looking for Earth-like biological markers. Lots of people looking out for signs of Dyson spheres or other biology-agnostic possible signs of life.