Because there were no antibiotics and advanced medical equipment in ancient times. So even a cold could kill you, let alone a sore on your back.
Let's start with the ancient medical environment. In ancient times, there were no antibiotics no penicillin, no saline, and of course, people still knew to drink boiled water. But most people don't have this condition, so in ancient times, a woman giving birth to a child is a ghost gate, people with wind chill, headache and fever is also considered a ghost gate.
The life expectancy and mortality rate of the ancients after they got sick was no match for modern people, and we should look at the diseases of the ancients from the point of view of a childhood with no vaccines and no antibiotics.
Again, ancient people had sores. A lot of ancient people died of sores, such as Xu Da, the founding minister of the Ming Dynasty, and Qin Hui, the great traitor, both died of sores on his back. The sores on the back were called, "hitchhiker's sores" in ancient times. If the ancient doctors can not name the name of the sores is okay, once the name of the disease, then it means that in ancient times the death rate is very high. Otherwise it wouldn't be known by so many people.
We now have people with hitchhiker's back sores, and with advanced medical care, aseptic surgery can be performed, and decaying tissue and wounds with inflammation can be removed. Aseptic sutures can be performed. But in ancient times this was impossible to achieve. After all, sterility could not be achieved in ancient times.
Even if you could remove rotting wounds in ancient times, you couldn't do it aseptically, so you'd still end up with an infection and die. This is why, the ancients were most afraid of sores on their backs. After all, it would rot from the outside to the inside of the body. The lethality rate is too high.
So, let's be more tolerant of the ancient people's health, it's not easy for the ancients.