As a rule, planets with life aren't all that rare in the galaxy, or at least not rare enough to be unknown... but there's a bit of selection bias going on here too.
Humanity keeps finding life similar to their own across the Orion arm because they're looking for planets they can inhabit without having to terraform it.
Most of the time if Humans don't find a world the can't live on, they don't spend much time or resources trying to colonize it. Terraforming is a long, arduous, risky, and expensive process that most people really don't want to bother with. It's much easier to colonize a world that comes pre-built with a native biosphere compatible with our Terran biology. The greater a xeno-ecosystem's biocompatibility, the more likely it is to have been colonized for ober 5 decades by the time the beginning of Xenospychosis.
However, the complexity, compatibility, age, and diversity of these biospheres rarely align well with Human requirements. Out of the 237 life-bearing exoplanets officially catalogued & surveyed by Humanity, only 11 of them had a biosphere both resilient and compatible enough to allow for Human colonization without significant technological, industrial, and scientific developments still yet to be fully realized in the modern day.
To date, no terraforming venture has been successful.
- Future History; The Great Galactic Land Grab
