Why do survivors and mist monster rifts appear near slopes in Save the World?

I’ve had similar observations. Survivors seem to pop up on slopes more frequently, which might be because areas without important mission structures are utilized for these spawns. It’s like the game sees those spots as available space to use for spawning survivors or rifts.

I’ve noticed that survivors often appear on slopes, and you can sometimes even construct ramps and find them spawning there. Mist monster rifts, however, usually spawn at cave entrances or other specific locations. Although on rare occasions, you might find them in open areas, it seems more random for mist monster spawns.