It's hard hauling a heavy backpack up and down mountains all day when you aren't used to it mentally or physically. A lot of people aren't used to doing anything difficult in the first place.
Some of them try to skimp on decent shoes or other necessary gear and that causes problems or injuries but they end up having to replace it anyway if they don't quit. It helps to have comfortable shoes, clothes that don't chafe, a warm sleeping bag, a light pack that doesn't rub your skin off, a shelter that will keep you dry, and a stove that works.