Dead By Daylight plays with the concept of eternal torture and suffering. While the injuries one withstands in the Entity's realm are bad enough to leave any rational person with PTSD for life, the survivors within do not have the pleasure of being left with mental scars of a horrible past. Instead, those scars of the past are relived every single day.
The survivors of Dead By Daylight are under constant and ongoing stress at a level that most people do not experience in a lifetime. Constantly being exposed to torture and death of not just themselves, but their companions as well will most certainly create a numbness about those who are not quickly driven to insanity. To accompany this, these people have had the most important factor to keeping sane in these scenarios away from them: Hope. There is no hope for these individuals. Some have been there for months, others years, others centuries. It does not take long for an individual to realize that they are truly trapped in an eternal cycle of torment. As discussed in my interview with Dr. Payne, there are three types of people in these situations. Those who quickly go insane, those who become so numb and used to the cycle that they are little more than a robot, and those who thrive in the environments and may even begin to enjoy the process. Traits such as hyper competitive natures, athleticism, and an optimistic attitude are traits that contribute to those who thrive, whereas traits such as anxiety would attribute to those who quickly lose sanity.
A big part of the tornment of the Entity's realm is memory. Your body may heal, but you will never forget the suffering. This will inevitably lead to crippling PTSD which would be the leading contributor of people going insane. Past a certain point, the average human simply would not be able to put up with the psychological warfare that they are faced with in the Entity's realm.