The only reason is hardware limitation. I imagine it's more difficult to load at any point in the game in a massive game due to how much is stored in your memory.
Let's say you're playing a game and there's 6 NPCs outside and they're doing their own thing.
If the game has a traditional save system, when you exit the save location it's normal for these entities to rest let their position. Maybe at best their properties (maybe they were wet because of rain) are saved.
But it's much easier to just not save any of this info and reload everything from scratch and only save your progress and location.