Gold doesn't oxidize, which means that gold-to-gold contact has no extra resistance.
In other words, it isn't about the bulk resistance (the plating is very thin anyway), but about the contact resistance.
Silver is used in places where the resistance to current flow along a surface matters, such as the interior of a waveguide or the outside surface of a high-power RF coil. In these applications, a layer of oxidation doesn't affect performance.
Silver is also frequently used on larger RF connectors, where gold would be too expensive and not durable enough. The mechanical sliding of the contacts and threads when assembled is enough to break through any oxidation.