In classic tempering, why does reheating chocolate after cooling not simply undo the whole crystallization step?

Answer: It leaves the stable crystal seeds that later set the texture

It leaves the stable crystal seeds that later set the texture ✓ — Correct. Reheating just below the melting point of the stable crystal form melts the less stable crystals while leaving the stable seeds. Those seeds then guide the rest of the chocolate as it sets, which is why tempering is selective editing rather than a simple warm-and-cool cycle.

It preserves every crystal that formed during cooling — This sounds close because tempering does preserve something, but keeping every crystal would leave the messy mixture that makes chocolate dull and streaky. The reheating step is tuned to melt the less stable crystals and keep only the stable seeds that give a good snap.

It warms so gently that the crystals barely change — This is a plausible guess because the reheating is gentle, but gentle does not mean irrelevant. The temperature is chosen to melt the less stable crystals while keeping the stable seeds, so a few degrees are enough to separate a full reset from a selective edit.