The case is going to be that they haven't done it yet, but that case could collapse as soon as they manage to really do it -- and perfect reentry and reuse, particularly of the reentry system.

Getting refueling to work is another big challenge.

Assuming that is perfect the performance of a reusable Starship -> Moon -> Earth mission is not that good, landing 3 tons or so, not far off from the Apollo lander. To do better it needs to be refueled on the Moon which has all sort of question marks. O2 can be found on the moon and that's a significant part of the "fuel". Maybe there is CO2 or CH4 or something carbonaceous in the ice at the poles, maybe there isn't. An H2 rocket is a better bet in that department. Tall and tippy Starship would do great landing and taking off from the Moon or Mars if there was a set of chopsticks to catch it, landing on an unprepared surface looks challenging.

These aren't really "Starship" problems, they are problems that affect any vehicle trying to go to the moon. Back in the 1980s I read columns in Analog Science Fiction magazines "Science Fact" columns that tried to say we were sold up the river by the Apollo missions and should have had a more ambitious program, but the round trip to the moon is terribly difficult and Von Braun's mission architecture made the impossible possible

A Mars mission would be a break from the SpaceX experience -- so far SpaceX has been about pursuing commercial opportunities in near earth space, not "exploration".

Rapid turnaround with a lot of hardware can get you to orbit or the moon, but there is no such thing as "rapid turnaround" to Mars where you have to wait 2.5 years for the launch window. 10 tries will take 25 years. Right now SpaceX runs everything by remote control from ground control which is a perfect strategy for NEO, probably good enough for cislunar except for the landing, but landing and takeoff from the Mars requires an autonomous system which is not just a technological but a culture change for SpaceX.