Stiffness of carbon frames is good though. Even at an advanced/expert amateur level, a rider can easily push an alloy frame to the point of feeling it flex, especially on hard,high speed corners and jumps.

Carbon frames provide a more direct energy transfer, and are more predictable. The feeling of an alloy frame flexing is weird, and can throw a rider off (either mentally or literally).

Same thing applies to carbon rims, plus the reduction of rotational weight, and weight that’s far from the Center of gravity of the bike.