A policeman sees a drunk man searching for something under a streetlight and asks what the drunk has lost. He says he lost his keys and they both look under the streetlight together. After a few minutes the policeman asks if he is sure he lost them here, and the drunk replies, no, and that he lost them in the park. The policeman asks why he is searching here, and the drunk replies, "this is where the light is".[1]

Science very thoroughly follows the drunkard principle, because there is no sense in looking at places were you unable to find anything. Science is looking for what it can possible see. It cannot see a bacteria under ice of Enceladus, so it doesn't even try. But it can see sometimes gases around Enceladus, so it looks for them.

https://en.wikipedia.org/wiki/Streetlight_effect

We don't just look for oxygen. For instance, there was some excitement a few years back when there was a tentative observation of phosphine in Venus's atmosphere.

To have a good biosignature, you want a signal that cannot be explained by anything other than life. Of course, that means that there might be planets with life that we do not notice, because all of the observables we have access to have an abiotic explanation. Not much you can do about that other than a bunch of science.

Detecting that a planet might barely support life is impossible to do remotely for our next door neighbor Mars, and we have rovers and orbiters sending high resolution images every day. For exoplanets, we're working with 1 pixel images. Not 1 megapixel, 1 pixel. So for now we can only look for things that would make scientists say whoa, how can that possibly happen?

Becomes easier to understand when you design a different search and estimate cost. Try it.