These flights do happen. They are essentially cargo flights for now, with a handful of passengers also flying in each direction. Several airlines are also operating additional transatlantic flights with commercial airliners that are cargo-only, i.e. no seats can be booked.

For now, the variable costs for having a crew and catering on board are all that needs to be covered by the incremental revenue generated from having a few pax on board. The flight ops itself is paid for by the belly.