By saying that people usually come back, Dad seems to imply that it was a fluke that Momma
didn’t return to him and Sal. It wasn’t something they could’ve predicted, and as Dad sees it, the Winterbottoms should proceed as though Mrs. Winterbottom will indeed return. In this regard, Dad seems to have processed and accepted Momma’s absence in a way that Sal hasn’t yet, as his experience of Momma leaving doesn’t color his perception of other people leaving. His reassurance gives Sal hope that Momma will come home, which seems misguided given that Sal has left several clues suggesting that it’s impossible for Momma to return.