Cargo isn't nearly as profitable as you think, unless you're talking about Asia to USA imports.

An entire 10 ft x 7 foot airline pallet can be had for around $7000 US or much less give or take (surcharges included) and you could cram at least 4 premium fare pax or about 12 coach pax in that same space. Do the math.

And there is far more handling and security required (that is paid by the airlines) than with pax.