People had to fight to survive back then. Troops had diseases but they were not sedentary. For most part, they were farm boys.
If we were to have a draft today, our conscripts would be like Gomer Pyle in Full Metal Jacket.
If we had a draft we could find the relatively few men we needed out of 335 million people, it isn’t like the entire male population quit being male and doing physical work and activities.