The pair fed their own premature baby just once a day in between 12-hour stretches at an internet cafe, the official Yonhap news agency reported.
Police officer Chung Jin-won told Yonhap they "lost their will to live a normal life" after losing their jobs.
He said they "indulged themselves online" to escape from reality.
The 41-year-old father and his 25-year-old wife were arrested in the city of Suweon, south of Seoul, earlier this week, five months after they reported the death of their baby.
An autopsy showed her death was caused by a long period of malnutrition.
The couple had become obsessed with nurturing a virtual girl called Anima in the popular role-playing game Prius Online, police said on Friday.
The game enables players to interact with Anima and as they do so, help her to recover her lost memory and develop emotions.