We have limited resources, so we can’t look at everything in detail all the time. We have to prioritize how we are looking for life, and we only have one example of where life can be found: Earth. So we look for Earth-like planets because we have positive-proof that life can exist there.

Imagine you were looking for a doctor in a hospital; you’d probably look for someone wearing a white coat with a stethoscope hanging from their neck, right? But anyone in the hospital could be a doctor; the guy wearing skinny jeans and a sweater walking through the lobby could be a doctor coming on/off their shift. But we know that white coat + stethoscope = doctor, so we immediately look for those signs first.