Because that denotes a skill level. It does not denote combat experience. Combat experience and your proficiency (that is, skill level) in a given form of combat are not one and the same. You can be extremely skilled in the use of a weapon, but not have any practical experience in applying that weapon to combat. Even so, you can wield the weapon/perform the techniques of the martial art (and believe it or not, there ARE modern-day military martial arts) with ease. It might not be as good as having actual combat experience with a weapon, but it certainly does not mean you aren't extremely skilled. And of course, if someone with only combat experience comes up against someone who has both combat experience and has trained in the use of a specific weapon (and not via the DIY style of training), then my bet is on the guy who has somewhat less combat experience but greater proficiency in their weapon. And you know what, if a dwarf has the right personality, not having combat experience might not matter too much-if they take joy in slaughter/are somewhat psychopathic, then they might actually perform almost as well as a battle-hardened veteran since they would not be bothered by what they see. The main problem in applying what you learn to practical use would be the fact that, let's face it, war is ugly. Dealing with the psychological part of the deal is the biggest obstacle; before the U.S. military revamped its training methods, a study found that only around 25% (might be underestimating it) of the men in the field would actually shoot at the enemy. This was back in WW1, of course; by 'nam, the ratio had gone up dramatically.
BTW, hunting was considered a way for nobility to keep their combat skills sharp in times of peace, so even regular hunting and non-notable kills should count towards combat experience.
What I said about civilians having military skills is a simple extension of what you suggested, pilsu. Eventually, armies are going to come from world gen and actual members of the nations, and potentially every member of a civ could be drafted as a member of the military. Therefore, if all their soldiers have adept skills, then by logical extension their civilian population would also have it. This goes double for when civs can keep growing after world gen, since eventually the soldiers of all nations would consist of NPCs born after world gen, who therefore spent some portion of their time in game as peasants (even if it was just 1 turn after they grew up). May I suggest an alternative? Have nations at war or on bad terms with other nations ramp up the training of their military, in proportion to the length of time the conflict has gone on.
I know about history, and also know that it would lead to the ruin of both nations economically long-term. But if you are able to manipulate events just right, you could manage to create two nations with strong warriors (or at least enough for your satisfication), arrange it so they are at peace, let them recover (during which the new elite men should be training up better-than average recruits), go to war with them AFTER they recover, wash, rinse, repeat. Alternately, give a militaristic nation control of an important point on a trade route-they should be able to expand pretty rapidly once they start benefitting from it (though that will have to wait until the caravan arc).
I actually think it would be a simple addition to the generator to at least make elite soldiers better able to make judgement calls for the purposes of attacking and retreating, based upon what they can see within their line of sight. And the idea of a scouting party would be based simply on how long it took for the "feelers" to be killed/how many of them were killed. BTW, I do believe NPCS will eventually be able to dig, so they might be able to accidentally hit your cistern/river that way.
Edit 2: That's why I used the comparison of someone who is both battle hardened and skilled in weapon/shield using vs someone with just battle hardening. BTW, part of the problem is also the huge bonuses to stats, and especially speed, that long-time champions rack up as they learn more weapon skills.