Industry standards classify fine jewelry by its precious metal content, and gold over silver delivers it twice: at the base and at the surface.
Elemental Detail
The convention of producing safety chains in the matching metal and karat grade developed from the same quality standards that govern fine jewelry construction generally. A secondary component produced in a different metal or grade would undermine both the visual consistency and the material integrity of the complete piece.
Made With Purpose
Two metals can share a surface indefinitely or reject each other slowly: silver and gold fall into the first category, bonding well during electroplating and expanding at rates that keeps the bond stable over time.