This is for two reasons. First, gold is extremely inert, meaning that unless gold is exposed to very harsh chemicals or harsh vapors, it will not corrode or oxidize. It will remain a pure, low-resistance conductor. Corrosion of connectors is often a problem. When you use a contact enhancer, be very sure to follow the directions, and avoid spreading enhancer about your equipment.
Home audio is almost always single-ended interconnects.
RCA plugs form fair to poor connections that degrade with time as corrosion works into the metal-metal contact and as the spring tension of the connectors relax. Gold-plating reduces the effect of corrosion and locking RCA connectors solve most of the mechanical problems. However, these premium phono connectors are rare and expensive. If RCA connectors weren't a de facto standard, we'd recommend against them. Unbalanced interconnect wires vary in geometry, material and price.