Because the engineering involved in what has already been achieved with the whole Starship program (the stage0/OLM quick disconnect/chopsticks, as well as the full flow staged combustion Raptor: v1, and v2, and especially v3) is a far greater technological achievement than any part of the F9/FH stack.

Starship is a HARD project, and even the components that they have completed are insane levels of engineering, far beyond even the whole F9 program.

It’s not just a “does the rocket work” thing. Every working part of Starship so far has been a monumental breakthrough of unprecedented scale. The fact that they built the largest ever orbital launch system (it’s currently not yet reusable but it can indeed put a ~hundred tons into orbit in a single 15 minutes) is enough to classify it as an unqualified success. Their stated goal just happens to be significantly greater than even that.

It has already surpassed the (non-reusable) Saturn V in price/perf as well as payload capacity (and development program total cost).