One reason for sure is that they DID experiance something. They honestly believe in what they are telling you.
But when it comes to see God - the Bible says: (Exodus 33:20) . . .no man may see me and yet live. . .
Our weak human eyes cannot see such glory and survive.
About reincarnation there are many evel persons that want to fool humans. They make them beliveve that they have lived before through little details that fits in. But here the Bible again is very clear: (Revelation 12:9) . . .the one called Devil and Satan, who is misleading the entire inhabited earth;. . .
He love to fool people and lead them away from the truth about death and God.