In reality, however, things didn't go that way.
In 1947, Mordecai was born in rural Alaska. Given that 1947 was the time the Cold War began, Mordecai and his family were dangerously close to their enemy. In his teenage years, he took advantage of it and snuck into Russia with his friends and led a guerrilla campaign. It ended in the late 1950s with the whole group being brutally murdered by the Soviets.
Rigby came along in 1964. He was born with a series of mental disorders that could not be treated in any way possible.
Sometime after Rigby turned 12, his family moved to Alaska, so that they could escape the cities. It was there, Rigby befriended the ghost of Mordecai, who returned to his hometown after death. His parents noticed something strange about him...Rigby would spend hours in the house or outside socializing with the spirit he became friends with. They figured that it must have been some kind of angel or imaginary friend, so they shook it quickly.
In his adult years, Mordecai and Rigby became annoyed by each other, eventually leading him to leave Alaska and move back into the cities.
In 1981, Rigby was shot and killed during the assassination attempt of Ronald Reagan.
In 1992, Benson managed a park, where he committed suicide to avoid going to prison for the murders he committed while working there. In the afterlife, Benson continues to manage the park, luring spirits there to work for him. Which is how Mordecai, Rigby, Skips, Pops, Muscle Man and High-Five Ghost found their jobs there.
The only one of them who was aware of his death and afterlife is Benson, who refuses to let anyone else know, because he doesn't want them to leave him alone in the park, which everyone had abandoned in 2011 due to paranormal activity.
Anyone who dares to enter the abandoned park is constantly tormented by the spirits, leading them to suicide so that they can live and work at the park too.