It is interesting that with so many advances in technology, and general knowledge of filmmaking etc that none of these sequels can begin to touch the original. There's just something about that movie. Part of it is simply the pacing of it imo, the director/writers know that there can be too much of a good thing and treated it like a thriller/horror flick, not simply an action flick.
In short - it's because making a genuinely compelling, "good" movie is very fucking hard.
There's no formula for it (or if there is, following it makes your movie feel formulaic), there's no budget short-cut for it, and it generally falls apart the more cooks you get in the kitchen (which is obviously an endemic problem in Hollywood).