Because a group shows something a one-to-one interview cannot: how a child behaves around peers when nobody is managing them. A prepared answer survives a solo interview. It does not survive twenty minutes of being disagreed with, ignored, talked over and asked to share a decision.
A secondary school taking a child through DSA-Sec is committing to them for four to six years, and committing them in return: an accepted offer removes a child from Secondary 1 Posting and binds them to that talent for years. Schools are choosing a member of a team, an ensemble, a squad or a committee — and the cheapest way to see whether a child can function in one is to put them in one.
There is also a practical reason. Group rounds are efficient. Where a school shortlists more than a hundred candidates, seeing five at a time is the difference between a week of interviews and a month of them.
And there is a third reason, which is worth naming plainly: the group format is harder to coach. Task content is often withheld until late — one school publishes that its sports tasks are made known only after the application period, and that its debating topic is released near the trial date. A round that requires other children, in real time, is a round in which a scripted child has less advantage.
One correction to a widely repeated assumption. It is often said that group rounds are there to assess collaboration and leadership. The one school-published purpose we could find says something narrower: CHIJ Secondary states that its group interview assesses commitment and motivation level. That may be specific to that school — but it is the only stated purpose in the public record, and it does not say what the prep industry says it says.