We also have samples of other nearby planets and astral bodies that do have signs of life. Signs, not evidence.
Mars, Enceladus, Titan, Europa, and some other places have signs that life might be present. Might be. Don't bet on any of them, but there is still a lot to be explored.
Because of our empirical experience and study of hundreds of thousands of organisms from all kinds of species.
>We have a sample size of one - Earth. It's laughable that we're therefore so committed to defining requirements for life.
We also have sample sizes of many other nearby planets and astral bodies with no signs of life.