Cigarettes are sold by the each, not by weight.
The tobacco companies have done many things to get folks smoking more and make more profits, not unlike most other companies.
Would anyone be surprised that some companies and cheaper brands put less tobacco in each cigarette, possibly adding additives to enhance the effects of the smaller amount of tobacco?
I have smoked many brands and the last 15 years just smoked filterless.
I did notice in my packing days and so did my friends that cheep off brand smokes were much less dense and after packing would revile much more empty space at the end. Premium smokes on the other hand really didn’t pack all that much. American Spirits hardly pack at all. And I don’t think a Nat Sherman would either.
So back to the original question.
People pack for several reasons.
Habit, Smoking is filled with ritual. Watch individual smokers they do many thighs exactly the same way. over and over again. Not just packing but they way they open it, what they do with the wrapper, turning over a lucky cigarette. Packing is just one.
Control the density of the tobacco as explained above.
And other reasons mentioned by other posters.