Silver is a better conductor than gold or copper. Gold is often plated onto other metals because plating is cheap and gold does not oxidize. Silver oxidizes, but, silver oxides are quite conductive too (though not as much as pure silver), so it may not make sense to constantly clean and polish silver connectors or binding posts. Also, when two metal surfaces are pushed together quite hard, like when you tighten down a spade, or squeeze in a banana plug, the points of contact of the metal tend to be gas tight so those contact points won'[t oxidize.