Probably because most stories ignore that part...
Well no, they typically end with the human dying and the god being left alone. I had just forgot because this story has been so cute and domestic recently.
Most don't show that part tho. They show the human and the god/demon/angel/immortal being fall happily in love and the last time we see them the human is still young and healthy, that the "happily ever after" leads to the human dying and leaving their lover behind is something we usually don't see. The overwhelming majority of stories that do actually show the human lover die are stories where the MC is the reincarnation of the original human lover.
Fuhhhhh I forgot about this part when it comes to those godxhuman ships….