Security groups have an existing purpose and authorization model. A User Administrator, for example, can manage membership of all security groups in the Microsoft Entra organization. The role might use groups to manage access to applications such as Salesforce. A User Administrator shouldn't be able to manage the delegation model itself, which would be the result if security groups were extended to support "resource grouping" scenarios.
Administrative units, such as organizational units in Windows Server Active Directory, are intended to provide a way to scope administration of a wide range of directory objects. Security groups themselves can be members of resource scopes. Using security groups to define the set of security groups that an administrator can manage could become confusing.