Part of it is narrow body v wide body aircraft. There is an active secondary market for narrow bodies and J* overwhelmingly operates narrow bodies. You would struggle to operating lease an A380.
In that case, why do Qantas own most of their 737s? Why would Jetstar even care about the secondary market? That would be more of a concern for the leasing company, not the lessee. And once again, what is the advantage in one segment of the group purchasing aircraft to then lease them to another part of the group?