Because high-purity silver is soft and easy to be oxidized, 925 silver is added with 7.5% other metals to make it have ideal hardness, which can be better shaped and inlaid with various gems to make various silver products.
By the same token, 18k gold (an alloy with at least 75% gold content) has higher hardness and toughness than platinum, and can be connected and tempered stably under various extreme conditions, so it is not easy to wear and deform. Moreover, the cost is relatively low, the recovery rate on the market is relatively high, and the inlay is firm, so 18K gold is still popular.