Lufia II, a somewhat obscure RPG on the SNES, flat out stated that monsters carrying gold was simply an abstraction of the whole process. In the opening cutscene, your character brings some monster carcasses to the item shop in exchange for gold. After that, your party simply receives gold when they kill monsters.
So, after playing Lufia II back in 1996, that's how I've always viewed the whole process. Monsters don't carry gold; it's just saving time by automatically doing the tedious legwork for you.