Sephiroth discovered that the Cetra were a nomadic people, so they never stayed at one certain place for too long. You're thinking that they traveled from planet to planet, but they really just traveled around the world.
Eventually, the Cetra quit being nomads and they made permanent settlements. Sephiroth mentions this inside the Nibelheim Mansion.
Okay, I get it. Sorry for the confusion. Thank you Xurts, that really cleared things up for me.
Maybe the lifestream is neutral itself but reflects the thoughts/feelings of others a person encountered or affected when they were alive. So if a person lives a life of crime, when they're in the lifestream, they feel all the things they've done to people, none of which are good. Likewise, a good person would only feel/know of all the good things they've done for people , which are good. It's an idea.