Because we are spiritual being having a human experience. If someone dies not having an opportunity to hear the gospel, they will have that chance in the afterlife, and have the availability to choose Jesus. Choosing Christ here when it's available will bring greater rewards in in heaven as real truth can only bless humanity even if they are unaware. That is why it's good to learn here, and okay for those who never heard the gospel while here.
My step grandmother went off her meds and started lying to police about everyone around her. My uncle disappeared to Hawaii, fell into drugs, and came back a complete shell of his former self, and stole $10K of his parents money to buy more. We don't trust either of them. If someone in my family went off the rails and suddenly had all the vitriol in the world for a faith that was no longer theirs and was extremely rude to their cousins or nephews/nieces about it, yeah. I'd teach my kids not to trust them either.