RCA plugs are not meant to be plugged and unplugged over and over and have the ground sleeve as security when connecting. This allows them to have pure copper or silver for the signal conductor. For 1/4" and 3.5" plugs, the most common connectors on this thread and used for headphone cables, there are no pure copper or silver connectors available. This is primarily due to the fact that pure copper and silver are too soft. Brass or bronze with gold or nickel plating are necessary to withstand both the number of plug/unplugs and provide the structural rigidity necessary for these types of plugs.
Also, a good number of DIYers make cables to save on costs, especially in this thread it seems. Triple digit plugs don't usually come into play.
3.5mm plugs are almost always gold plated because gold doesn't corrode anywhere as easily as copper, not because other materials are too soft.