We don't, because Starship has not had even one successful flight with any appreciable payload. It's absolutely possible that the booster will need to be completely redesigned, and become much more expensive, in order to achieve the mission goals.

It's also worth noting that a captured booster has only once been successfully flown again - and certainly not in the kind of tight time line that the in-orbit refueling operation requires (first flight was March 6, second flight was October 13 - and no more flights are planned anyway). There is currently little proof that boosters can be "rapidly and fully reused" as needed to match any of the cost promises.