Outside an amplifier, the two single wires that form the loudspeaker cable run closely together for several meters. Compare that to what goes on inside an amplifier. The wires run 250mm and two in different ways, one for ground and one for output. The distance between the wires is perhaps 100mm inside the amplifier and 2mm outside the amplifier. The current in wires creates a force between them, which is proportional to the length of the wire and inversely proportional to the distance squared between the wires. So the force is perhaps 25,000 times stronger in the loudspeaker cable outside the amplifier. It is clearly very different conditions, so the cable should not be the same.