This is rarely mentioned. Because of its design, Starship can't just go to the Moon, it first requires orbital refueling (from other Starships). NASA estimates they would need at least 15, but that was before the Starship payload capacity was downgraded.
It's actually mostly about weight in disposable rockets. Stainless steel is heavier than the usual aluminum-lithium alloys in conventional rockets, or the carbon fibre that's increasingly being used like Rocket Lab's Electron rocket. It's unnecessarily heavy if you don't care about returning an upper stage back to Earth.
Stainless steel was actually chosen for Starship for good engineering reasons, not idiotic cosmetic ones like Edolf Muskler's cybertruck debacle. The specific alloy that SpaceX is using handles extremely low temperatures very well, so it's good for tanks for liquid methane and liquid oxygen propellants. But that specific alloy also handles the high temperatures of atmospheric entry very well, which reduces the needed mass for a heat shield. Aluminum-lithium alloys and carbon fibre don't handle reentry heat well. The original concept for Starship was actually carbon fibre, but after they did the math on the mass of the heat shield needed, they realized that they might as well just go with a variant of plain old stainless steel. It's easier to work with than specialty alloys or carbon fibre. And by aerospace standards it's also dirt cheap.
Fun fact about stainless steel rockets: WD-40 was invented to protect the first US ICBM from corrosion in humid conditions. "WD" stands for "water displacement".