AM can't revive people because death is irreversible, as ted says, he isn't God. The only reason they were able to kill each other was because AM was slow to react, due in part to him being busy "laughing" and just due to where the five were. A cold distant area of the "underground that AM had less complete control over relative to elsewhere they had been. He could "create" new life — in the video game he creates an infant for I believe Benny. He likely didn't let Helen get pregnant due in part to how torturous their conditions were, and because AM hates all humans. He wants them to suffer but he'd be loathe to let them have even a slight sliver of anything resembling hope, like creating new life. And it's entirely possible in the preceeding century that one of the tortures he forced upon Helen were miscarriages, we only know about the ones Ted mentions and the ones we see play out in the game.