If demons really would exist, they would've slaughtered us all by now!
Some "possessions", like the one of Anneliese Michel, look scary as hell, but hey (like Cofty mentioned), many generations before us were superstitious as hell, so it might be still a bit in the blood, so to say (and Hollywood thrives on it).
A possible explanation could be found in the realm of psychiatry (see wikipedia's section below):
In June 1970, Michel suffered a third seizure at the psychiatric hospital where she had been staying. She was prescribed anti-convulsion drugs for the first time, including Dilantin, which did not bring about immediate alleviation. She began talking about seeing "devil faces" at various times of the day. That same month, she was prescribed another drug, Aolept, which is similar to chlorpromazine and is used in the treatment of various psychoses includingschizophrenia and disturbed behavior; in her case possibly spiritual delusions. In depression by 1973, she beganhallucinating while praying, and complained about hearing voices telling her that she was "damned" and would "rot in hell". Michel's treatment in a psychiatric hospital did not improve her health and her depression worsened. Long term treatment did not help either, and she grew increasingly frustrated with the medical intervention. Being a devout Catholic, she began to attribute it to demonic possession. Michel became intolerant of sacred places and objects, such as the crucifix.
Michel went to San Damiano with a family friend who regularly organized such pilgrimages to "holy places"—not officially recognized by the church. Her escort concluded that she was suffering from demonic possession because she was unable to walk past a crucifix and refused to drink the water of a holy spring. Both she and her family became convinced and consulted several priests, asking for an exorcism. The priests declined, recommended the continuation of medical treatment, and informed the family that exorcisms required the bishop's permission. In theCatholic Church, official approval for an exorcism is given when the person strictly meets the set criteria, then they are considered to be suffering from possession (infestatio) and under demonic control. Intense dislike for religious objects and "supernatural powers" are some of the first indications. Michel worsened physically and displayed aggression, self-injury, drank her own urine and ate insects. In November 1973, Michel started her treatment withTegretol, an anti-seizure drug and mood stabilizer. She was prescribed anti-psychotic drugs during the course of the religious rites and took them frequently until shortly before her death.
In 1984, the bishops made a petition to the Vatican regarding the exorcism rite and a commission passed on the decision that she was mentally ill, not possessed.