Aren't your players interested in playing with each other's characters?
The players are. The PC's aren't necessarily. Players consciously doing heavy lifting to corral their characters into doing what will be fun for the players is the kind of work against the game system that we used to do a lot of, that AW games have been otherwise sparing me.
Aren't they interested in the same parts of the world, and therefore bound to interact around those things anyways?
Nope, different parts.
I am purposefully ignoring all the things the game already says to do to encourage these things, I am just wondering on like a basic level, what is the deal with a playgroup that starts out with characters in a mutually-intertangled, self-generated situation (Hx, starting in the same geographic place), and then reliably disentangles themselves to pursue disconnected ventures elsewhere in the fiction?
The deal is just that: they are entangled, and disentangle themselves. The Touchstone hies off on a separate mission and gets captured, and the others don't go get him because they're dealing with other Threats which are unfolding based on what honesty demands and what the prep demands. The Maitre D's restaurant gets burned down and she ends up in the hardhold, co-opted with her crew at risk, spending her time focussed on wooing the NPC hardholder and figuring out the NPC social landscape. The Brainer's body is there, but occupied by the Hoarder, who figured out how to use the psychic maelstrom to unseat people from their bodies; the Brainer has gone off into the landscape of the psychic maelstrom itself, to the extent that we decided just to drop that character and give the player a new PC out of one of the NPCs who were physically present. While the Hoarder and the Maire D' are physically in the same hardhold, they're no longer aligned, they're pursuing different agendas, and they're simply interested in different aspects of the setting, and not even really interacting with the same NPCs.
I throw threats at them, but that doesn't necessarily unite them, if one decides to rally and face the threat, the other to get the hell out of dodge...
It just sounds like crazy talk to me. I mean, this isn't Sorcerer, where the game sets up situations in isolation... your players (and you!) have to be working pretty hard to not follow up on any of their Hx, or need each others' skills or assistance, or exist in the same location interacting with the same NPCs...
Following up on Hx is fine, but what does that mean? You saw into my brain when I was sleeping, I fucking hate you for it, it means you can interfere with me better, and so I have all the more reason to stay the hell out of your way. In-game, if the characters need skills and assistance, they're going to get it from whoever they can get it from -- they don't care if it's NPCs or PCs. With no scarcity of room, what's to keep them in the same location?
But leaving that aside, the game also explicitly tells you to make PCs who are allies, if not friends.
Who says they stay allies? AW has no status quos.
In a harsh, post-apocalyptic world, there are plenty of problems that are going to make PCs need each others' help.
Or just as much reason to run in different directions, or avoid each other, or sell each other out and leave each other stranded, or just not have the safety or resources available to offer help ("hell if I'm going after HIM")
It explicitly tells you to create NPC triangles, which bind the interests of the PCs together in complicated ways. It suggests a world of scarcity in which one cannot actually exist in isolation, regardless of how much empty space one can find.
That last bit is what I'm saying: scarcity of room. "One cannot exist in isolation, regardless of empty space." Right: so the space around the PCs should be empty, so that their choices are indeed "each other" or "isolation". Whereas if those spaces aren't empty, but thickly populated with threats and resources, then "isolation" does not equal "isolation from the other PCs".
So I mean, it seems like what you mean by 'scarcity of space' is more like 'scarcity of fiction' -- or more accurately, density of fiction. And the game already does a lot to push in that direction. Even inertia is on your side: you have to do so much more work as an MC to create separable fictional worlds for every PC to explore, in isolation. It's so much easier to reuse connections, NPCs, places, groups, and doing so is going to encourage interaction.
On the contrary, in AW it is WAY less work to make more fiction than it used to be. Maybe when I was a GM-as-storyteller, having to draw up the maps and stats and relationships of the next hardhold over, all in advance, would have been a disincentive (except for the part that I loved the activity of making them). AW allows any PC to hie off over the mountains to a new hardhold and all I have to do is barf forth some apocalyptica -- eeriely silent broken roads, animal corpses in postures of pain, lights on in the ruin of the Wal-Mart, and then say "you see someone you know in the window. Who is it?" and we're off and running. There is nothing to stop you from spinning the world as big as the characters make it, so that whenever the PCs wander away from each other, unless you have a mandate to turn them back towards each other, if you're just playing to find out what happens and saying what honesty demands and being the PCs' fan, you can just follow them in different directions across a rich, thick world.
There are lots of reasons you might not have encountered this problem. You might be irritated by the activity of coming up with new settings and antagonists and stuff, rather than excited by it, and so just naturally and instinctively have, as you say, "reused". Your players may be naturally team-oriented, and their PCs may fit Vincent's original conception of a Firefly-like community of allies who fight amongst themselves but unite in the face of threats. The social contract of your gaming table may be such that when one PC refuses to go rescue another people frown and go "hey, come on", rather than chuckle in delight and nod in appreciation of the fictional rightness of that choice.
Anyways, it's cool that you've identified a thing that is not happening in your game, naming a new principle to guide your play is a good way to try and keep it in mind. But I think the tools to enact that principle are already present, and it makes sense to focus on bringing them to the forefront.
I think the tools are indeed there: making it a principle is about permission. Otherwise, when the character hies off over the mountain and I'm like "...uh... it's a big desert, nothing there, you should go back", or whatever, I feel like I'm acting against the stated principles of "being a fan", "telling what honesty demands", "playing to find out what happens", etc.
Another way to say it would be to alter the agenda to be, instead of "make the characters' [individual] lives not boring", rather "make the characters' lives together not boring".
I'm also curious if your players notice this, or feel it is a problem; do they want to have scenes with each other, but feel like they've become trapped by 'what their character would do', or what? Are they just literally so geographically isolated that it no longer makes sense for them to ever interact? Also, do you use Fronts, and if so what is your Front creation like in these games?
The fact that you say "trapped" there suggests that perhaps we have some slight difference in creative agendas? How about "seduced" by character realism, or "committed" to it, or "feel annoyed that they have to choose between honoring the fiction of what their character would do, and fulfilling the social agenda of playing together"?
The players and I find each individual scene fun, but notice to our dismay in the aggregate that they're playing with each other less. We're spending more time as spectators to each others' stories -- eager spectators, but still, increasingly spectators. Or else doing what feels like violence to the fiction in order to correct this problem.
I have always had this problem as a GM. long before I became an MC. "Keeping the party together" has always been my bete noire. AW feels like it actually made this worse, by discouraging the usual railroady methods of pushing the characters at each other. It's true, it does offer other tools which could be adapted to this purpose; but I think the principle is required to activate them, because, in the game as published, "keeping the party together" is not demanded by the rules, by honesty, by the prep, or by the principles.