Why? Because they confuse their interpretations of Genesis with God. For example, this comment from a post this past weekend,
If Genesis is not true, the rest of the Bible isn’t true, Jesus never existed, God doesn’t exist, and this life is nothing. Because without Genesis, evolution comes into play and this life is utterly meaningless.
What happens if this person hits a wall when for a moment, he accepts some fact of science. He then loses all faith. He becomes an a-theist. His former fundamentalism is not changed, but the foundation is. This is only one danger in YEC. It removes Christ.