In moment of inertia calculations, "dr" is not explicitly included in the integral because the relevant distance is the perpendicular distance from the axis of rotation, which is represented by "r." The mass element "dm" incorporates the volume element and density, allowing for the integration of mass distribution over the object. While "r" may vary for different mass elements, it is treated as a constant in the context of the integral, as it represents the fixed distance from the axis of rotation for each infinitesimal mass element. Understanding this requires a grasp of multivariable calculus, which is often beyond the scope of introductory physics courses. Thus, the general formula for moment of inertia is accepted as is, with the understanding that it accounts for the distribution of mass relative to the axis of rotation.