Why is a punt calculated from the line of scrimmage?

All passes, runs, and kicks are calculated from the line of scrimmage. It is the one arbitrary line that can be used as a universal point of reference without subjective decisions, and thus can be universally applied. The exception is the field goal; it is calculated from the location of the ball when kicked, except that if there is a penalty, it is still calculated from the line of scrimmage.