[Tardigrades](https://en.wikipedia.org/wiki/Tardigrade) can survive vacuum, low temperatures, and moderate radiation for quite a while. They're *multicelled* organisms. How tough is the toughest hypothetically viable *single-celled* [extremophile](https://en.wikipedia.org/wiki/Extremophile)? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, so all precautions are taken to avoid that.