it's because they know they will fear the answer.
Religous belief becomes part of a person's identity. Mucking with identity is extremely painful, even though identities change throughout life.
being confronted with the possibility of changing one's identity based on outside factors (not, say, becoming a parent by choice or picking up a new hobby/sport) will cause defensiveness and evasion. This is why believers go through apologetics and other mental gymnastics to answer what should be simple questions.
Now you may close the thread. :-)