Cargo doesn’t quibble about crowded conditions or expect special treatment. Cargo doesn’t get rowdy and problematic in-flight. Cargo doesn’t require cabin crew to cater to its needs. Most importantly, cargo pays well.
A battle for weight, in other words.
Marisa Garcia has done an excellent job going into what makes cargo so interesting for airlines. You should give the article a read.
Passengers represent a net profit margin of $6.00 each to airlines. Using the median weight of a person at the world average of 62 Kilos (136.7 lbs), then adding the average approved weight for carry-on luggage of 23 Kilos (50.71 lbs), the average person who only travels with carryon luggage represents a weight burden of 85 kilos (187.39 lbs).
According to IATA’s most recent report, current global air freight yields $2.40 per kilo. The average passenger with traveling only with average carry-on luggage can represent a cargo-yield loss of $204 to the an airline—hardly a good exchange for those $6.00 of profit per seat.