Well, I'm with you that maybe a person from one world could run a game in another world, but I actually think it's pretty clear why you can't train a successor from people who died on their own.

Not everyone who dies in this whole world ends up in the Underground, right? It's not crowded enough. So I'd buy it that keeping your souls in the Underground, where you'd have to be if you wanna act as this "successor", takes a little something extra. Like a game.