After all, the term only applies to life on other worlds, not to the planets themselves. Imagine how much easier it would be to colonize an Earth sized planet in a favorable orbit around another star if it were barren of life. A life bearing world would pose many problems for the would be colonists. While the chances of alien microbes causing disease in Earth based life would be very unlikely, the chance of alien life producing substances toxic to us could be quite high.

The thought of traveling for decades on a one way journey only to find a planet infested with alien life producing toxins that make the environment unfit to live in is a very scary proposition. It would be much easier if the planet were a clean slate from which Earth lifeforms could be introduced.