Why do creationists bury their head in the sand when asked a reasonable question?

Because they don't have the answer for it. Usually, though, they answer it with the phrase "God did it," "We will never know Gods ways," or my favourite, "That just proves you are a moron." Also many who do give up (which is more often than one would expect but not as often as they should) are not trained in the area they are discussing. There are some people, however, like Nephilimfree and GunneLPercher, who claim expertese in every subject known to man and will never give up even if every scientist on the planet tells them that what they are saying is impossible.