Segmentation enables reusability, performs better, and results in smaller/simpler pieces. F9's booster is a prime example. A lunar lander designed to only handle the LLO-to-lunar surface roundtrip is reusable. Some sort of LEO-LLO roundtrip ferry is more difficult but less complex than SS.Segmentation means no meaningful reusability. I think you're missing the whole point of Starship/Super Heavy.
Even a second stage segment that only goes from the booster to LEO and back is easier a universal second stage like SS. It only has to deal with the Earth's atmosphere, always lands on a prepared surface, and has a lower reentry velocity.