The Inspiratory Reserve Volume (IRV) refers to the maximum amount of air that can be inhaled forcefully after a normal inhalation. The reason why you can't measure the IRV directly is because it is difficult to accurately measure this volume using standard spirometry techniques alone.
Typically, spirometry measures lung volumes and capacities by having the individual perform specified breathing maneuvers into a spirometer. However, accurately measuring the IRV requires inhaling forcefully after a normal inhalation, which is not a standard maneuver measured by spirometry.
The procedure you mentioned, which calculates IRV as Inspiratory Capacity (IC) minus Tidal Volume (TV), offers an indirect way to estimate the IRV. By subtracting the TV (the volume of air inhaled and exhaled during normal breathing) from the IC (the maximum amount of air that can be inhaled after a normal exhale), one can estimate the IRV.
It is important to note that although this method is an estimation, it is a practical and commonly used approach to estimate the IRV without requiring additional specialized equipment or techniques.