[Kromm]

First, if you want an army, you have to pay an army! An army wants as much per person as a party of delvers – and maybe more, if they're numerous enough to pose a serious threat to ordinary civilians and their property, and can therefore hint that something bad might happen if they're left hanging around town with idle hands. That's the power of collective bargaining, dungeon-fantasy style.

And why would someone pay an army that rate? Because the idea of "rank" or "level" or "point value" is totally invisible in the game world. Sure, the delvers know they're more competent than Joe Spear Carrier, but their sponsor really has no way of knowing they're x times more capable, so their sponsor isn't going to pay each of them x times what they'd pay Joe. Likewise, Joe isn't going to charge less because he secretly knows he's terrible and not worth the price.

So it comes down to sponsors saying, "I'm seriously underpaying for this work because, frankly, I can't afford the army it would take. Is there any smaller group confident enough, perhaps overconfident enough, conceivably foolhardy enough, to take what I can afford to pay for the job? Or just to work on spec, because I hear there's, um, treasure. Yeah, that's the ticket."

There are exceptions, of course, when the job very clearly calls for specialists. A lot of quests pretty much need something like "a couple of champions aglow with the power of Good, a professional slayer of demons, an expert on magic, and someone who can defeat the trap that killed the last 100 people to try." An army can't cover that ground. The sponsor might not have any real guarantee that the delvers are capable specialists, but there's usually a way to establish that they at least have the right specialties, and then the sponsor will be inclined to pay them extra for that, which probably still won't be as costly as an army.

Also to be considered is the fact that there aren't infinite numbers of people available for silly quests. Most people have families to feed and lives to live, and take up arms only when pressed into service during times of war. Going off into the demon-, dragon-, and undead-infested wilderness is either too dangerous or not a steady source of income for hundreds or thousands of men-at-arms. So the would-be men-at-arms mostly become porters and farmers and smiths and so on, because piddly pay every day at a safe job beats bigger payoffs on rare occasions at a job where people die.