A pipe is uniform throughout, while a limb is not.
Work Step by Step
A 1-meter length of pipe has uniform mass density. By symmetry, its CM is at the geometric center, the midpoint. Your arm and leg aren’t uniform; the cross-section differs, and the density of muscle, fat and bone isn’t the same. Generally, the CM of your arm or leg is closer to the torso than the midpoint of the limb.