When you start the car, the engine needs to overcome the inertia of all the parts that need to move in order to run. This means that the engine needs to produce a lot of torque, which requires a lot of energy. Once the car is running, the engine does not need to produce as much torque to keep it going because the parts are already in motion.
Additionally, when you start the car, the fuel injector needs to inject more fuel into the engine to help it start up. This extra fuel is not needed once the engine is running and has warmed up.
