The wiki says that rotational inertia is defined for point-masses, and by extension continuous bodies. It says:
This simple formula generalizes to define moment of inertia for an arbitrarily shaped body as the sum of all the elemental point masses $dm$ each multiplied by the square of its perpendicular distance $r$ to an axis $k$. Of course, we can use that to calculate the rotational inertia for discreet-distribution bodies. We can also use that for continuous mass bodies if we treat the differentials as point masses.
While the standard area and volume differentials are small enough to be approximated as points. How will you approximate a cylindrical shell as a point (we use cylindrical shell differentials when we find the moment of inertia of a rod with the axis of rotation being the axis of symmetry of the rod)?
This is the rod I'm talking about: