Labour law voids a contract clause where a worker freely agreed to fewer holidays for higher pay. Why does the law override a genuine agreement?

D. Because the employment relationship is structurally unequal

Freedom of contract assumes rough equality of bargaining power, and the employment relationship is defined by its absence, so the law makes minimums non-waivable. If they could be waived, competitive pressure would push every worker to waive them, which is the whole point of a floor.