I think you'll also find the cargo bays of most of those aircraft stuffed to the gills (especially since the lack of SLF means they don't need to limit the cargo load to stay below MTOW).
Under normal conditions, cargo is in many ways more profitable than passengers - and while I doubt they are showing a profit only carrying below floor cargo, they can approach break-even. Particularly since the provisions of the government aid means they need keep most people on the payroll.