Why do runners cut back their training in the weeks before a big race?

Answer: Tiredness fades faster than fitness, so they race fresh

Resting makes their muscles grow bigger right before the race — A few easier weeks are too short to build much muscle, and that isn't the goal. The gain comes from repairing tiny muscle damage and refilling energy stores.

Running less helps them drop weight and race lighter — Runners often gain a little weight during a taper, because muscles store extra carbohydrate along with water. That stored fuel is helpful on race day.

Tiredness fades faster than fitness, so they race fresh ✓ — Hard training leaves fatigue that masks a runner's real fitness. Fatigue clears in days while fitness fades over weeks, so cutting mileage for one to three weeks lets the body recover and refuel without losing much fitness.