why do the humbermans insist that Liesel call them mama and papa

Liesel was basically an orphan and alone in the world. Her father was killed, her mother had left and her brother had died. The Hubermanns wanted to give Liesel a sense of place, a feeling of home and belonging. Liesel's parents were also outlaws of the Nazis and Hans wants to keep this a secret and avoid exposing Liesel to danger.