I'm not sure they do?
Most have a stove - but some like Trangias (meths), some use hobo stoves (wood), Some MSR or optimus (any liquid fuel) some prefer gas (err gas)
Most wear jackets - some like ventile, some goretex, some wax cotton etc.
Most wear hats...leather, felt, even flat caps or watch caps.
Most use rucksacks...I could go on
So yes most people use carrying gear, cooking gear, clothes, stoves etc. These things are needed in living outdoors. The only think that generally I find similar is a wide brimmed hat (although I don't wear one myself), flat cap and proud of it.
Similar question would be why do all racing drivers dress similarly and drive similar looking cars...cos its sensible for the activity in hand
Red