In real life there are more than 2-3.6 billion planets that are considered habitable.
It is not that these planets have life, but that they have atmosphere, water and stars that emit radiation similar to the Sun.
These planets will possibly develop life, perhaps millions or billions of years in the future.
But back to the subject they world could have been terraformed by humans before the Imperium, for humans to inhabit them, terraforming and placing plants and oxygen and that is why some planets are as similar to Earth as Cadia.
The Adeptus Mechanicus had Terraforming technology for the 41st millennium, in the book Cybernetica it is mentioned that Admech has technologies to even create magnetic fields on planets generating an artificial nucleus.
It is mentioned that humanity discovered the Warp-Drive in M18 and in M24 the Age of Stryfe began.
This means that it took more than 6,000 years for humans to terraform more than 1 Million Planets, taking into account that DAoT's technology is more advanced than that used by IoM.