What may be happening is this. When a raquet is new, the graphite fibers are in their unstressed, natural condition. The spacing between the fibers is natural / unstressed. Then during restringings and play, graphite gets compressed onto itself. The strings pull inward. There may be a "compaction" effect. Graphite fibers may be becoming "denser" with age. I would think that this compaction may lead to the the loss of flexibility and side-way resilience in a frame, compared to the time when the fibers were newer and farther apart from each other.
Also, plastics (== grommets) become stiffer with age, not softer.
Same will happen with silicon or foam (if any was used inside the frame).
Same with leather on the handle.
The effect is probably more pronounced the more "soft" infusions a frame has. Fiberglass, twaron -- all those "softeners" probably compress more than graphite itself, and probably lose their softness faster.