The Falcon 9 booster is going relatively slowly, so retro propulsion into the atmosphere (re-entry burn), the grid fins, and the landing burn are sufficient to

The Falcon 9 booster is going relatively slowly, so retro propulsion into the atmosphere (re-entry burn), the grid fins, and the landing burn are sufficient to land it. Starship is intended to re-enter from orbit, which is a lot faster. Using aero braking requires much less fuel, and avoids the need for a re-entry burn which may not even be feasible.