It's not the grit as much as the character of the finish. Many platers do offer mechanical polishing, ie glass bead, or vibratory with polishing media (steel, glass, plastic), but you never get the same luster that buffing with rouge offers. You can't plate then polish because the thickness of the plating is less that the depth of the roughness.

Also consider that electroplating is a field effect, and the metal ions are attracted and stick to the nearest high spots, so when there's an imperfect surface the peaks get over-plated (chrome burn), and the recesses get underplated. That's why you'll often see a yellowish tinge on insided corners of plated objects.

As for galss beading speeding up the job, it might in theory, but in practice the buffing wheels work so fast that it wouldn't save much if any. If you ever visit a plating shop, you'll see how much the operator has to keep the object moving to keep it from being overheated and blued by the polishing compound.

BTW- if you're ever in a bike shop that has "Kingsbridge" cone wrenches, you'll see about the limits of chrome on a mechanical polish. The chrome has good luster, but you still see the surface character of the steel. The ne effect is sub par if you were plating a frame.