Because that would imply that the grand parents are the bread winners in the family, instead of "enjoying their golden years".
It's like that pop-psi tale about the guy who never knew what chicken breast tasted like. When being a child, his mother reserved the breast for the father because "he's the one who keeps us fed and clothed". Later when he was a grown up, his wife reserved the breast for the children because "they deserve the very best".