This was a conscious decision that is kind of based around necessity of gameplay more than anything else. I think I wrote about it in some depth back when the game was altered to work this way, but here's a summary off the top of my head since I'd never find that.
Sidebar: There is a hardcore scouting mode (galaxy map option)
This more or less works like you want, because I figured some people would want that for thematic or other reasons. But it only has a radius of 1, IIRC. It's a much harder mode unless you have a very cross-connected map, in which case it can add some thematic richness and just a different style without being TOO much harder. So this would be one thing to experiment with.
Note: I'm open to other modes
But nobody has really asked about them or used the main one much, so... eh, there's been plenty of other things to focus on.
Note: To me, most things can be justified in sci-fi
Thematically speaking, it could be "resonance waves triggered from nanobots that are expanding out through the galaxy, in a vaguely von neumann-like fashion."
...Actually, the idea of this being a von neumann machine of tiny scouts is super plausible. I was just going to make up some technobabble and then realized that's a super plausible explanation and kind of how scouts would work if they were built that way. It would go out in a wave based on past scouting, if small bits are left behind and build new ones. But not everything would go out evenly, because available material and other happenstance (distances, etc) would cause that to be uneven by nature.
Soooo... actually thematically the current approach can be really plausible! Huh. Who'd have thought it would really be that reasonable. I felt like I was tossing out thematic elements when I was making it, and just thought of this now off the cuff.
Actually about the mechanics
All right, so this is kind of what I wrote before, or at least the gist of it, which is in no particular order but hopefully follows logically:
1. Manually scouting (for free, when it doesn't cost you something other than wall clock time) is Bad, because people will prioritize doing that early in the game. The only thing that stops them from doing that in other RTSes is that the other player is rushing them. But in AI War titles, there's nobody rushing you at the start; you're at your most stealthy. So really it was just a time tax, or a micromanagement tax, in the first game and earlier versions of this game.
2. Having the ability to manually peek at stuff is important. Hence us adding the hacks to send nanites. So basically this lets you spend a finite resource (hacking) to pay for your scouting. It thus takes away the direct challenge (the hacks are easy and instant), but has a meta level cost to it. This keeps your decision to do it and your actual execution of your decision close together, which is important to me at least.
3. The rest of the scouting is something that is really about seeing more of the map over time, at a rate that isn't really supposed to be that variant. Aka, there's never a case where (at no meta cost) I want person A to see the whole map silly early. That takes away all tension. And there's no point where I want person B to be blind super late in the game, because that's just frustrating. Therefore, seeing the map is really a function of progress through the game, and it's meant to be revealed at a pretty consistent rate over time... except "time" is irrelevant in AI War.
4. Wall clock time, or even simulation time, doesn't really progress the campaign. It progresses things like CPA countdowns and wave timers, but not much else. The idea is that frequently you need to be able to stop your progress through the campaign and just deal with the CPA for a while, then rebuild and resume the pace of the actual campaign as you like. So there's this other "time" sense in the game that isn't measured in time at all. It's actually measured in AIP.
5. Sooooo... the most natural thing to do would have been to say "as AIP goes up, you get to see more." But not all AIP is created equal! Sometimes you lose a valuable capturable and it costs you AIP, and... you'd get scouting for that? That doesn't make any sense. AIP is generally the measure of your progress through the campaign, but things like those and also things like AIP-reducers make it not entirely linear.
6. The only thing that is at least somewhat linear is planet capture, as that happens on a semi-predictable schedule in most campaigns, though different rates for different players. So tying it to this made sense. I think at the time I also had the idea that maybe we were basically gathering some intel from the AI command station as it blew up. But that doesn't work as well as my more recent thought, since it doesn't just get intel on AI planets.
7. One of the things that's good about having it be tied to planet capture, and then just push the general frontier of the galaxy overall, is that you have plenty of chances early in the game to go "okay, out of what I can SEE, what's the best thing to capture." You don't have to also wonder "should I be looking for an even BETTER target to capture? You know what, maybe I'd better scout the whole map before capturing anything, just to be sure."
8. This also needed to be fairly topography-neutral, though. There are a lot of map types, ranging from snake (only 1 new planet possible at a time from adjacency alone) to spokes or similar (literally a frontier of multiple dozens of planets after a certain point in the game, so nothing is more than 4-5 hops "deep" from you, compared to being 100-120 or more hops deep from you in snake). So we introduced a minimum and maximum number of planets it would explore on the frontier, to keep it from being vastly harder or easier to play map types based on the scouting mechanics. They already are hard enough to choose between based on defensive and offensive options.
9. One reason why I made there be NO adjacency bonuses at all in the main system (stuff goes out along your frontier at random, IIRC, or very close to random beyond the direct adjacent planets) is that I didn't want "where my scouting is happening" to be yet one more thing that is tied to the location of the planets you capture. It's already really hard to choose a planet to capture, based on if you can defend it, what it will cost to defend it, what you capture, what it might put you on the path to capturing, and so on. Adding in "things I can just basic style SEE" on top of that was too much, in my opinion. Analysis paralysis would set in hard. "Do I capture this worse fleet over here and get to see a ton more, or take this amazing fleet over here but not get to see much?" You'd really be trading in AIP for scouting in some cases, as in adding extra AIP for certain scouting tasks, which felt unfair to me.
So yeah, that's the explanation of why it is the way it is. But as noted, I'm open to other modes.