Habitable, in science fiction and perhaps in general usage, has generally meant somewhere people could live without too much technological assistance; makes for interesting stories but I'm not convinced it's a good idea to colonize any planet that has it's own life, even if that life is deemed to be 'primitive'. Although most speculative fiction seems to treat as given that we would do so, it generally starts with it being relatively easy to get there (ain't gonna happen), that an equivalent to photosynthesis would have made Oxygen abundant in the atmosphere (no reason to actually think that would be likely) that people would be relatively resistant to poisons, allergens and infections from alien life (pure optimism) and wiping out whole alien ecosystems to replace them with more human friendly ones is both ethically sound and the smart thing to do (a holdover from past times when nature's abundance was there entirely for human exploitation, was seemingly endless and if you couldn't eat or otherwise find an immediate use for living things, you killed it). Myself, I suspect the scientific value of alien life in an uncontaminated state would outweigh the value of potential new real estate even if we could actually get people to star system 20 light years away and - luckily for the lifeforms there - we can't. It would be a huge major project to get a small robotic probe that far and we still have some rather serious problems to deal with closer to home that could use all the technological and other resources we can muster.
Outside our solar system there are several ways that life could be detected - eg by anomalous atmospheric compositions if our telescopes get good enough - free Oxygen for example - or by the EMR emissions of technology using aliens. Those need not be deliberate; our own use of radar and radio makes Earth unusually bright in some bands - mostly those used by FM radio and VHF television. Noticing the brightness isn't so hard but actually decoding it's content at interstellar distances would be. There are some especially powerful transmitters that could get noticed such as the Aricebo Observatory's radar. An alien receiver looking our way could find a source billions of times brighter than the sun at that wavelength. Other ways of finding non-terrestrial life might be collecting debri with interplanetary or interstellar origins that have remnants of non-terrestrial life in them. Some claim fossils in meteorites that have Mars for an origin. Presumably big impacts on Earth have sent debri into space that have remnants or signs of Earth life on them. Damouse, Sending probes may not be technologically impossible but they would be very costly, take a very long time and run a high risk of failure. Or are you disputing that we have serious problems closer to home that could be considered higher priority?