The interviews aren't all 1-on-1. The need for the CBP employee to electronically open the conditionally-approved applicants' files one by one and modify them one by one is there.
CBP even says this:
For each family member that you wish to add, you must create a GOES User ID and fill out a separate application. For family members that are under the age of 18, you will need to provide a legal guardian who must be 18 years old or older. When you call the Enrollment Center to schedule the interview, arrangements can be made to interview the entire family at the same time.