This is because one netherite ingot needs to be applied to a diamond gear to convert it into netherite gear. Furthermore, diamonds will be needed to duplicate the netherite upgrade smithing ...

After crafting a netherite ingot, place it, netherite upgrade smithing template, and the diamond armor item to be upgraded on a smithing table to upgrade it to netherite.

Having Netherite Ingots, the players can start upgrading the Diamond gear. To do it, Craft the Smithing Table using 2 Iron Ingots and 4 Wood Planks. Place the Netherite Ingot in the 2nd slot.