Why is joining the Fajr congregation as a voluntary (Nafal) prayer disallowed if a worshipper has already completed their Fajr alone?

When Congregation Begins during an Individual Prayer

• Because Fajr has no Wajibat.
• Because performing voluntary (Nafal) prayers after the Fajr obligatory prayer is legally disliked (Makruh).
• Because Fajr must be prayed in three units.
• Because the Imam is always a traveler.