One softball answer to that is that any civilization that doesn't get off of its home planet is likely to go extinct, so there's less chance of it actively modi

One softball answer to that is that any civilization that doesn't get off of its home planet is likely to go extinct, so there's less chance of it actively modifying the environment in ways we can detect. It's also less likely to need to develop long range, high powered communications that we can detect. At the moment I think the idea that we're only looking for life that fits some definition of success is specious, we're looking for any life we can find! It happens that a big, spread out civilization that looks like us is more likely to emit something we can see, be it atmospheric, electromagnetic, etc, just by nature of being in more places in space and having a larger footprint. There are astrobiologists who spend a lot of time thinking about what life may look like, but when it comes to where to put limited resources, we start with what we know to look for (us), and go from there. You might be interested in the book 'weird life' by David Toomey, which tries to theorize about what some more exotic living things might look like in the galaxy, and expounds on some of these definitional questions.