Rolex made a deliberate decision when creating the Day-Date in 1956: it would be their flagship, their statement of ultimate achievement, and therefore it would only ever be crafted in 18-carat gold or 950 platinum. This exclusivity is intentional—the Day-Date in steel simply doesn't exist and never will. This policy elevates the Day-Date above other Rolex models and ensures that anyone wearing one is wearing precious metal on their wrist.
It's Rolex's way of maintaining the Day-Date's position as the "president's watch." In the pre-owned market, this means every Day-Date you encounter is genuine precious metal, though it also means Day-Date models command higher prices than steel sports watches. The investment is significant, but so is the statement.