But what's behind this assumption?
Superficially, it seems silly that a horde of goblins or gnolls out on a raid would value coins at all. After all, what would they spend it on? No human town would trade with them. Desperate raiders just look to get by, day to day. They'd be much more likely to loot ornaments, clothes, weapons and armor, supplementing their own lack of crafts, and of course they'd value food, drink, slaves, livestock and edible corpses. But going around with a pocket full of pennies like some kid on the way to the grocery store?
Here are a few interesting ways out:
2. Individual humanoids don't find coins useful, but their leaders do. Individuals only carry coins when the whole tribe is on the move, to lighten their load. A tribe-level economics of brute force prevails beyond the pale of civilization. To save face and lives, confrontations that would likely end with both sides weakened are decided instead by a contest of dominance, and an offering of coin from the weaker group to the stronger. The weaker group sends an expendable member hauling the coins to a distant place, where the stronger group goes to collect and claim victory over the one unfortunate, while the main body of the weaker group makes an orderly retreat. Humanoids have found that humans also respond to extortion or tribute, and savvy adventurers know what to say and do to set this kind of negotiation in motion.
3. Humanoids do have an economy, just not as we know it. Behind the front lines of the wilderness is a shadowy market in arms, provisions, and the finer things in life, run by renegade and half-blood traders. The amount of negotiable coins found in any one humanoid purse or lair is highly variable, because this market produces its own tokens out of bone, shells and sand-glass; some amount of the humanoid's "wealth" will in fact consist of these trinkets. It is a very obscure corner of the black market, indeed, that will let you convert these finds to human currency (and, of course, at coppers on the gold piece.)