Because that's how the strongest men before us trained. Properly performed, full ROM barbell exercises are essentially the functional expression of human skeletal and muscular anatomy under a load. Balance between all the muscles involved is inherent as the muscles contribute their anatomically determined share of work.
Wouldn't disagree with any of that other than if you wanted to be picky Barbell's haven't been around that long (mid 1800's). I believe Pavel said that Kettlebells have been used in Russia since the turn of the century. Farmers or perhaps blacksmiths have traditionally been the strongest men. Training like them would be a great way to get "real world" strong
"It appears that, originally, kettlebells were counter-weights used in Russian markets. Country folk started throwing them around and eventually they became very popular in Russia as a training tool. In 1913 the Russian magazine Hercules reported "Not a single sport develops our muscular strength and bodies as well as kettlebell athletics." A Russian strong man was referred to as a kettlebell man --girevik. Girya is Russian for Kettlebell. Although kettlebells develop strength, a kettlebell-trained body is not bulky."
Kettlebell History and Research
I bring this up because while they are new here (US) they are not new in Russia. Kettlebells are essentially a tool, very similar to dumbbells but superior in some ways. Barbells, dumbbells or kettlebells will help if you use them correctly.