Why is the threshold for completing prayer not considered equivalent to the minimum threshold for Jumu'ah prayer?

Primary text

The Jumu'ah prayer requires a full Rak'ah because congregational prayer is a condition for it, and thus reaching the minimum requirement (one Rak'ah) ensures that the worshipper does not miss the predominant condition of the congregation. This differs from the mandatory quantity in regular prayer, where the quantity is determined by its essential commencement (Takbirat al-Ihram).