Because they happen to be the ones that have players attached at the moment.
That other party of NPCs you just met in the tavern, they could have a story too and it might even be more interesting than yours were it ever told; but none of us will ever know because those characters don't have players attached and thus nothing they do is ever likely to become all that relevant. An analogy is that you're watching a Liverpool-v-Arsenal football match and because Liverpool is your team (analagous to being the PC party in a game) you're going to follow their story even though you're well aware there's well over 100 other clubs in the FA (analagous to all the other adventuring parties out there in the game world).
As for the "wargame" aspect: I'm not quite sure how to phrase this but I think there's a big disconnect between a) trying to tell the story in the here and now of play via forced narrative and-or railroading and-or hard scene framing and b) just playing the game organically, following along with the in-game decisions the PCs/players make, and letting the story in effect write itself in hindsight.
Put another way (and maybe not any more clearly!), is the table approaching an element of play - a big scene, a search for an item, an interaction with a guard, whatever - with a specific view as to where this action fits into The Story, or is the approach a more here-and-now-based one of this is where the PCs have taken themselves and this is what they find themselves doing at the moment?
Me, as both player and DM I far prefer to just play the game, with or without any sort of overarching plot line, and let the story - in whatever form it may take - see to itself after the fact. Put another way, we do what we do in the here and now; and when we read over the cumulative game logs later a story (or more likely a whole bunch of interwoven stories of scale both grand and small) will emerge.
Lanefan