Since the temperature limit of available materials is the fundamental limitation (even after making custom high-temp alloys), this allows them to maximize mechanical power from the turbopumps, which raises performance.

We might imagine a conservative FFSC design which accepts very low temperatures in exchange for making it easy (low R&D cost) to reach high longevity. Raptor is not a conservative design, so it requires more R&D to achieve that longevity.

https://www.youtube.com/watch?v=twnZYPdFgbU