It's probably because of how the game works... So if you saved during a missions and screwed... And saved... You would be screwed every time, and need to restart from the beginning. Some people forget doing multiple saves. I remember playing Tomb Raider II, and that was one of the first games where you could save anywhere. I once panicked while she walls falling off a cliff, and saved lol. So yeah, when I reload she would fall to her death everytime. So, I had to restart the level over again lol. "Good Times"
It also probably the way enemies spawn and stuff... Imagine booting a save up and there's enemies around you and kill you everytime... It would break the game.
From: TheBlueStig | #007 You want a tough FPS to get through, try playing through Project I.G.I. where you can only save at the END of the level, and each level is bare minimum 45 minutes to an hour long.
For a game without respawning enemies, I feel it hurts the game. I'd like to be able to save before some of the cooler missions so I can redo them later. Also wanted to unlock stuff and leave outposts and create a save for a messing around game save.