Specifics vary depending on jurisdiction, but in most cases the only people who can travel with the cargo are either associated with the airline itself, or with the cargo (eg. livestock attendants, security guards).

To carry revenue passengers would require having to staff flight attendants for the flight and a different set of safety requirements for the cabin.

Furthermore, there are different cargo safety regulations for CARGO ONLY AIRCRAFT or PASSENGER/CARGO AIRCRAFT. Adding a single passenger would restrict significantly what can be carried and how it can be packaged.