Race style bikes stiffness is about performance efficiency.

aluminum structures survive longer if they don't flex.

or like Airplanes, flex cycled below a replacement threshold.
they measure hours of service, then park them in the Desert..

Want stiff for sprints, and less stiff century rides?
maybe you can seek out a frame like Richey, or others.
main frame Aluminum, rear triangle in Carbon fiber composites,
as well as the fork..