Why would you keep having kids?

maybe the woman was of the old fashioned sort;
she was a stay-home wife, who dreamed of a life of being famous, but didn't have the guts to do something about it and her husband was the one making all the money for the family.
the few times he was home, they probably got on it like rabbits and possibly their religion didn't support the prevention of any offspring coming from the sex.

The movie was set back in the day when contraceptives were not commonly used. Families, especially those as rural as this one, also had more kids to help out with various chores/tasks around the farm. Large families were pretty common back then, as well.