Because the art of an unplanned dismount from a bike typically ends in you going up and over with a raft of machinery following you, typically landing on if not close to your head. Landing on your head off a bike has a high probability because of the mechanics of the starting position, similar to that of toast always landing jam side down when droped from a worktop.
Id hazzard a guess that fell runers typically go straight down on there arse seeing as they dont have a large round pivot point infront of them called a wheel, trying to send them up and over.
So there a perfectly valid reason to wear a lid on a bike, regardless of speed or terain!