When all the plausible answers have been disproved, the answer must be sought outside our understanding of plausibility.
I posit that humanity is suffering from a disease that affects the nervous system and brain function. I've been calling it "Stagnation".
The same way nature has always used diseases to weed out over-populated species, it has infected susceptible humans with a pathogen that makes changes to the pathways and working functions of the brain, which in turn, distorts perceptions. I've been calling this disease Stagnation because, over time, the brain seems to become very inflexible in how it operates and delivers perception, unable to take in enough new information to support brain plasticity. This appears to cause a stagnation of thinking, a limited pool of thought that is constantly recycled and reused regardless of the situation.
Stagnation appears to be a type of pathogen that dampens or re-programs brain structures in humans, over time, impairing cognitive functioning and Perception. It affects people of all ages, though children often show great resilience in fighting it off. It affects people across genders, races, cultures, and socio-economic positions. It is contagious through exposure.
According to Wikipedia, Perception is the "organization, identification, and interpretation of sensory information in order to represent and understand the presented information, or the environment."
All signals go through the nervous system, but Perception brings those signals together with learning, memory, expectation and attention. All of this processing happens completely outside of conscious awareness. Perception arrives in our conscious mind, already formed, waiting for us to apply our judgment, understanding, and decision. Perception is our own, personal Reality. To think it might be affected by a dysfunction of any kind feels like the biggest threat to our identity. In fact, it feels like the threat of death. That is one reason people with Stagnation cling so tightly to the reality that feels real for them. They are not able to effectively process anything more, which can itself be terrifying.
Learning, memory, expectation, and attention, are the very functions affected by Stagnation.
A person affected by Stagnation experiences negative brain effects when the disease progresses, reducing capacity for rational judgment, cooperation, impulse control, and empathy. At the same time, these impairments increase the likelihood of fast judgments, outbursts, impatience, and violent actions. In this diminished state, most people resort to a reliance on rote thinking patterns rather than thinking through every decision and judgment on its own merits, as a coping mechanism. They surround themselves with elements, information, media and people who most closely mirror their view of reality, which, like never using a hurt muscle, results in further atrophy in the areas of learning, expectation and attention.
Humans are more susceptible to Stagnation when they experience extreme and/or long-term trauma. Humans who daily experience pain, anger, fear, disappointment, grief, hunger, discrimination, and other traumas, put their nervous systems under strain. In this condition, they incur a higher risk of contracting Stagnation as a secondary condition. However, many humans succumb to stagnation without a clear cause.
Also susceptible are humans who carry significant Power for long periods of time. People carrying the heavy weight of Power, or engaging the stressful behaviours required to maintain it, over-stress their nervous systems, allowing Stagnation an entry-point. When people with Power get Stagnation, they have more ability to use money and influence to force the world to look and feel the way they perceive reality, to reduce the threats to their perception. Therefore, Stagnation in a person who wields Power is particularly dangerous to other people. The fear people feel around Power makes them susceptible to catch Stagnation themselves. The decisions and use of power are initiated from a position of impaired judgement and perception.
The deterioration in key brain functions experienced by people with Stagnation causes them to focus heavily on self-serving perceptions as real, and threatening perceptions as false. As Stagnation progresses, people may come to see opposing views as an enemy of their truth, evoking primitive emotions of anger, fear, tribalism, and jealousy. Wide-spread, these tendencies in populations lead to war, unstable social situations, unsafe living conditions, random violence, and institutional violence. These mechanisms eventually eliminate large numbers of humans, while creating the conditions for others to die through diseases, malnutrition, starvation, or other deprivation-based causes. Nature employs Stagnation as a strategy to reduce our numbers.
Because so many humans are now infected, it is not clear whether the tide can be turned. It is difficult, but not impossible, to renew the plasticity of the brain through exercise, exposure, attention, learning, and memory techniques. Simply entertaining an opposing view as possible for just three minutes a day will improve the effects. Brains are remarkably resilient when we decide to engage their programming actively. Unfortunately, the very effects of the disease render the victims resistant to both diagnosis and treatment. With so many of the world's most powerful deeply infected, it seems unlikely that the humans can avoid the widespread devastation nature has in store.