Chris,
As Rutty explained, jackets and gloves are part of the game, no different to lycra and carbon fibre bikes for cyclists.
Shooting jackets evolved slowly over many years. The earliest were worn to protect your clothes from dusty or muddy firing points, at a time before washing machines. Then pads were added for your elbows, and shoulder for military cartridges. Only later did the material get heavier; leather was used because it's hard wearing, but it stretches so a canvas lining was the next step.
It's important to note that rifle weight increased before heavy jackets became the norm. Jackets reduce fatigue, but that's a response to heavy rifles (0.22 Smallbore) not a cause.
Padded gloves are just a simple protection. A 14lb rifle on bare hands is not pleasant, especially when a sling is used in Prone and Kneeling.
Special supportive Pistol vests were used in the' 70s and '80s, but I believe came on the scene quite suddenly, rather than slowly evolving, so were banned.