Extra super accounts are mostly an accident of employment. Before November 2021 a new job with no fund nominated meant a new default account, and a career of four or five employers could produce four or five accounts. Millions of Australians hold more than one, and most did not choose to.
The cost of holding them is not the headline percentage. It is the flat part of each administration fee, charged in full on every account regardless of how little is in it, plus any insurance premiums still being deducted from a balance nobody is contributing to.