The USA requires spirits to have a minimum of 40% Alcohol by volume. However, I can not find a reason of why this is the case. Other countries allow for lower volumes. For example, in Mexico, most Tequila is 35% and in the USA, the same brand has to export the same Tequila at 40% which changes the flavor drastically.
Simply, it's a legal requirement to differentiate between different types of alcoholic beverages for the purposes of taxation, licensing and permitting, and labeling requirements. In the U.S., this is set by the Department of the Treasury's Alcohol and Tobacco Tax and Trade Bureau and their definition of a distilled spirit includes the requirement that it be 40% ABV, though that is not the only requirement, so not are all alcoholic beverages over 40% ABV are by definition distilled spirit.
As you've noted with the example of Mexico, this is not the same definition global, only for U.S. legal purposes. The E.U., for another example, has yet another set of definitions.