Why explain “al-birra man āmana” as the piety of the believer?

Qur’anic Ellipsis (Ḥadhf): Implied Words, Relationships and Rhetorical Brevity

• To relate righteousness to its bearer
• Treating birr as a title for the believer
• Taking man āmana as a conditional result
• Deleting birr and retaining only man