Now, how many supervising FIs are actually present on the ground at the time the student is sent solo to make that a worthwhile signature?
'Supervision' is not defined. One or more Supervisors must be listed by the ATO/DTO with the CAA, as the responsible* person/s to sign the paperwork. There is no requirement for the supervisor to ever be present but only to inspect the instructors paperwork and sign it as required. This is not unique to aviation.
* my word