Two thoughts.
1. As far as I've heard, MGS4 had a feature-length opening. Since you can stylise something in-game with every minute detail perfectly as the game designers/producers desire, compared to potential conflicts over adapting to film, I don't think the MGS fans would appreciate it as much.
2. Target audience + history of VG movies failing. Sure, MGS is a more realistic series than say Mario, but that doesn't mean it can't fail on the big screen. If they remove any critical element of the game (example, cardboard box), then it won't arrest the niche audience at all. Also, what non-video gamer would want to watch a film on a game that talks about espionage? (Actually, the answer to that is quite a number, but it's not the target audience...)