The MGS games's stories consist of cut scenes and the radio conversations right? Overall, the whole story is around 5-6 hours. If you take some of the unnecessary conversations and scenes out you get at least 3 hours. Way too long for an action movie, and you can bet they will dumb down the story to something like 2 hours of rushed, confusing plot.
Next look at the metal gear universe as a whole. Just take the first MGS. You play as a clone of the greatest soldier ever who must infiltrate a terrorist compound, kill a guy with psychic powers, a cyborg cloaked ninja who happens o be his best friend who he killed once before, a muscular guy who cant lift a fighter jets machine gun, a military fighter chopper, a tank, a giant walking talk, oh and not to forget his own twin brother.
Now tell me how you would portray all this without making it look cheesy? (and that's just the first game. How would you explain that a hand can posses a body?) Keep in mind that a metal gear movie would need a huge budget, something that Movie companies are unwilling to give to video game movies (look at how the Halo movies has stopped, and Halo has better chances of success at a box office than MGS does). The fights between Ninja and Snake, and Snake and Metal gear must be especially well made. Now consider how shit all video game adaptions have been (Alone in the dark ,silent hill and Resident evil were awful) , why will a Metal gear one be any different?
No, I'd rather they didn't do it and kept the game as it is. The only way it would work is if they release it as a series of Anime OVAs