The real reason is that chocolate is applied on a conveyor belt and extra chocolate drips onto the bar. Not all of the excess is successfully blown away so you get that vein in the middle.
I just looked it up, and they actually have a machine specifically there to add the vein. Why waste money on a machine that adds veins on the top of a long chocolate bar(A futa bbc).
I've even gotten a whole bag that didn't have the vein, so I see that they really do have a machine for this, and that they were experimenting with preferences for whether their bbc(big bar o' chocolate) has veins or no veins.
Enlighten me!