Random.insideUnitSphere doesn't work as expected

Does anyone know why this snippet returns a position around world space zero? (the transform is a child)

 float radius = 5;
 float zDistance = 5;

 targetPosition = new Vector3(Random.insideUnitSphere.x * radius,
                      Random.insideUnitSphere.y * radius,
                     zDistance)
                     + transform.position;