Nothing says they can't. Just like they can also just hide or flee. A standing military you only use once every 30 years would be a huge expense, one many societies could not afford.
Hobgoblins and Goblins train cavalry, have soldiers and legions, etc. They form armies that are explicitly mentioned in the Manticore write-up. Orc armies(also known as hordes) are cliche at this point. Tritons are noted to have waged many wars. You can't do that without armies.
Yes, it absolutely makes sense. Armies, even Orc armies, don't just wander across harsh terrain willy nilly. They travel down easier paths and roads. Halfling villages are hidden in out of the way spots where armies don't go and down paths that even Rangers can't find by looking.
Exactly. Which brings us back around to monsters(true monsters, not orcs) being so rare that they are just not much of a threat to society. Adventurers(PCs) just encounter an abnormal number of them due to how the game works.
Of course it makes sense. They're predisposed by their size to not fight, so they generally flee from danger whenever they can. I mean, all those wandering halflings probably started wandering for a reason.
Generally you tend to have areas of the world that are far more dangerous than normal (e.g. the Sword Coast) and plonk your adventurers down there.
They run and rebuild. Or they die. Exactly what most human settlements have done throughout human history.
