If someone was hoping to inherit from parents after the second parent's death, it might be unnerving to see the parents separate their lives to the extent of one moving in with another partner. There would be some likelihood that the new partner would inherit on the parent's death, and that the new partner wouldn't subsequently bequeath that to the 'stepchild'.
Of course that also applies if a bereaved parent remarries. I have seen it in friends and family. One relative didn't inherit a penny from his parents until he was 77 - because his widowed father had remarried to a younger woman, who lived to be 97. At 77 you can't use and enjoy your inheritance like you would if you were younger. And yes, it does seem unfair.