You cannot plate anything over everything. That's why ENIG on the PCBs consists of gold over nickel. It won't hold on bare copper. The same with connectors, there is nickel layer underneath the gold.
AIUI, gold plates onto copper quite well - the problem is it alloys relatively quickly due to migration of the copper and (above a couple of %, if memory serves) the alloy becomes a worse conductor than pure gold (and, above ~20% Cu, worse than nickel). Hence the intermediate plating of nickel, which acts as a barrier to electromigration( & also adds a degree of wear resistance).