Why is it necessary to append a Surah in every rak'ah of a voluntary (Nafal) prayer, unlike the Fard?

The Necessary Acts of Prayer (واجبات الصلاة) Part 1: Recitation

• Because voluntary prayers are physically shorter.
• Because every two rak'ahs of a voluntary prayer are juristically treated as an independent prayer module requiring complete recitation.
• Because voluntary prayers do not contain sittings.
• Because the Prophet ﷺ always prayed them sitting.