Why UPS drivers don't turn left and you probably shouldn't either

It might seem strange, but UPS delivery vans don't always take the shortest route between stops. The company gives each driver a specific route to follow and that includes a policy that drivers should never turn through oncoming traffic (that's left in countries where they drive on the right and vice versa) unless absolutely necessary. This means that routes are sometimes longer than they have to be. So, why do they do it?