Because in the wake of story driven games taking up almost the entire single-player market, "try this part again" has become so much of a default assumption tha

Because in the wake of story driven games taking up almost the entire single-player market, "try this part again" has become so much of a default assumption that its exclusion would inevitably become a focus point. And the other big element, a strong element of randomness, that's really just secondary to permadeath, because permadeath without random would be a tragically repetitive slog (unless you make memorization the entire point) and random without save/retry would be kind of wasteful, as players would rather proceed even further than restart. When you don't aim for replayability, the xkcd random number implementation ("4") would be as good as any.