I was reading about the sea dragon rocket and was wondering why rockets today aren't made of cheaper materials like steel? And could you make a steel rocket today?
The "propellant fraction", or ratio between the mass of propellant a rocket carries and its total mass, is a very important factor in the rocket's performance. Lightweight materials such as aluminum alloys allow the propellant fraction to be maximized, leading to a smaller and overall cheaper rocket even though the cost of a given amount of material might be higher.
Steel is still in use in some rockets; the solid rocket boosters used on the space shuttle and planned for the SLS are steel-cased, for example. In this case, because the mass of the casings is discarded relatively early in flight, the cost benefits outweigh the mass penalty. Even some modern upper stages (where weight is critical) use steel; the Centaur's steel walls are so thin it has to be suspended from above or kept pressurized at all times, otherwise it would collapse under its own weight.