The Shooting rules are written from the point of view of units of troops firing upon other units of troops. A character moving around on his own is treated as a unit consisting of one model. In this respect a character is a viable target just like a regiment of infantry.
However, in reality a lone individual would be likely to escape the notice of enemy on the battlefield, strewn as it is with stragglers from destroyed regiments, wounded making their way back to camp, and the inevitable confusion and debris of conflict. The following rules represent the fact that characters are harder to shoot at than larger units of troops.