Because of the amount of travelling they do and the increased sweating due to over excitement. Or stress, the level of insensible perspiration is raised above the normal levels that are needed to control body temperature at rest (homeostasis). 

This amount of sweating  without being aerobic causes the body to over compensate by storing sodium in the skin. This is the start of most of the problems associated with dehydration and is one of the hardest to reverse without  the use of the patches.