Well, to be pedantic, there are many reasons. They range all the way from comedy, right through to just being a stickler for accuracy.
edit - looking at it again, jcd748 failed on accuracy as the earth does not lack inertia but rather possesses it in great quantities. 8.04×10^37 kg·m2, to give a rough figure.
edit - looking at it again, jcd748 failed on accuracy as the earth does not lack inertia but rather possesses it in great quantities. 8.04×10^37 kg·m2, to give a rough figure.