Just a handful of extremophiles can survive on the thin-atmosphered planets. They stick out like a sore thumb. As you would notice a spot of lichen on a concrete wall, the probes will notice them.
Earth-likes, on the other hand, are a big mess of tens of thousands of species. Every square meter of a meadow has a dozen plant and another dozen invertebrate species, not to mention bacteria, protozoans and other micro-organisms. It's a big mess and the simple probes that we shoot at planets just see it as "Yup, that's a lot of life here" unable to discern what lifeforms exactly are present. It's a long, hard work seeking out and documenting all the species in a transect for a human being, a short-lived probe equipped with simple cameras, spectrometers and radars has no chance of doing that.