Their goal is establishing a Mars colony (1,000,000 humans). That necessitates a fully reusable rocket that can get humans to Mars and back. That means being ab

Their goal is establishing a Mars colony (1,000,000 humans). That necessitates a fully reusable rocket that can get humans to Mars and back. That means being able to enter Mars atmosphere at high speed and landing on Mars. That also means using a fuel that they can manufacture on Mars to return to Earth. - Falcon doesn't have a reusable second stage. Starship's second stage is reusable. Starship should be more cost efficient because of this. - Falcon doesn't have the capacity to carry enough humans and cargo to Mars. It is a trip that can take upwards of half a year. Starship can supposedly carry 100 people and 100 tons of cargo. Falcon with Crew Dragon can only carry 7 people a few days. - Starship is designed to be able to enter Mars atmosphere. Its skin is stainless steel and some heat shielding. The stainless steel can survive much higher temperatures which means less heat shielding. Falcon is aluminum and would require more heat shielding. - Falcon has fuel tanks for its Merlin engines that use rocket fuel (RP-1?) and oxygen. The tanks are a specific size for the burn ratios. The rocket fuel can't be readily made on Mars and its mass makes it infeasible to carry enough fuel for a return. Starship's engines (Raptor) use methane and oxygen. Methane can be readily made on Mars. Changing Falcon from Merlin to Raptor engines would mean redesigning the tanks to have different sizes. Given all this, it makes more sense to design a new rocket than trying to redesign the existing one.