Because either Rey’s parents were on that departing ship or they’re dead and buried on Jakku. It can’t be both.
And Kylo can’t be trusted.I already asked this, but are we really sure we see Rey’s parents leave in that ship? Maybe it was just a random ship and Rey imagined her parents were in there, because that would make it easier for her.
Given Rey’s response to Ben, it’s clear that Rey has maintained a myth about her parents in her mind. Why can’t she have manufactured a memory? Not that unrealistic. Indeed, there are still Star Wars fans who maintain that Luke’s first go at the grappling hook missed when SW first hit theaters.