Passenger planes can't really be compared to cargo planes - they can be completely similar in most ways, but being ultimately designed for a specific purpose can make a huge difference, and from what I can tell, most passenger planes have half the carrying capacity or less.

Some airlines have converted passenger planes to keep them going, but they can't use the space the same way a dedicated freight plane can, and I think most of them carry priority goods rather than regular mail  / parcels.

If there were profit in it for both companies, it would have been done, I suspect.