The games would not translate well to the movie media. They would work better as television shows or a miniseries. There are too many bosses, in any FF game, to forge into an effective movie without cutting too much.
Take FFI. Even if you minimize the dungeons, take a look at how much is left in the game that you would have to address. The inital battle against Garland and the building of the Cornelia Bridge could be an entire episode of television itself. You would still have Astos, Bikke, Matoya, the Four Fiends, Bahamut... Each of which has their own story and buildup.
Trying to put them all into a single movie would either involve cutting a lot, or have it be a bunch of disconnected and nonsensical boss fights, which would wind up oversaturating the movie to the point that you lose interest.