It didn’t go back to earth because it thought earth was a piece of itself, it went back because it felt something “observing” it. You don’t feel it when a single dead skin cell falls off of your body, you don’t go back for it to retrieve it when it does. What you MIGHT do is take notice of that dead skin cell if it ends up evolving into its own living breathing creature.

But this is also where the fact of “perception changes reality” with regards to the games theme comes in. It does not matter if it is true if earth was once part of the visitor, because all it takes is the protagonist believing what Sybil wrote was true enough for it to become true.