Why is it that dwarfism is legally defined by height?

In the United States possibly in other countries as well, though I am not sure a person is legally considered a dwarf if he or she is under 4’10 in height. Why is this so? Dwarfism is caused by one of a number of medical conditions that may result in the shortening of bones or disproportionate growth of bones or even imbalanced hormones. According to Wikipedia, anyway. Considering this, why is it that a short human, who is lacking in any of these conditions, may be considered a dwarf? Why aren’t the criteria for being legally classified as a dwarf related to medical conditions rather than solely height?