Why would wild creatures in a fantasy role playing universe, such as bears, bats, tigers, sharks, or flatworms, regularly carry currency to the extent that adventurers would see them as a reliable source of dough?
The short answer is that this is just how the System works.
Let me explain. (In story form, because... well, I'm bored, that's why.)
Magic in the most raw, unstructured form is highly chaotic and has a tendency to destroy things. Imagine a universe like our own where the laws of physics are the only thing that keeps everything running smoothly. Something happens to introduce magic into the world and things start breaking down. Unless something is done to curb the effects of magic the world, and everything on it, are doomed. For example if the contamination extends as far as our sun then it only takes a minor disruption to cause a catastrophic event - a coronal mass ejection would be sufficient, but all it takes is a very small change in the strong nuclear force to make our sun either collapse or explode... or both.
Fortunately a group of scientists discover ways to manipulate magic and do useful things with it. Eventually they figure out a way to create a magic framework that takes raw magic and channels it in various ways. They can't just make it disappear, it can't be stored (because eventually boom, that's why), it has to be actively expended. They work on the problem for a few years and come up with the System: a magical framework that converts raw magic and channels that power into safe and occasionally useful outlets.
Over time (for reasons that only a few of the major gods currently are aware of) the amount of raw magic contaminating the planet increases. The System has had to grow to handle larger amounts of power and provide more outlets for that power. The engineers who created the System added all sorts of extras to the system, but when they found a way to connect the System to everyone they figured out fairly quickly that this would be a good long-term solution. They tinkered a bit and came up with some additions to the System that let everyone use some of the excess power to improve their lives. They added mechanics for all sorts of things to encourage people to burn the processed Mana more effectively: skills, spells, abilities and even Divinity. Anything that would (relatively) safely consume the power before it could cause problems.
Of course the engineers, being human, became the first gods of the setting. What's the point of creating a cosmic power machine if you're not going to get to use it, am I right? A few of them kept working on the System, but the rest mostly got drunk on the power and just started playing.
Each time the power levels reach a certain threshold the current group of System Lords tinkers some more and adds more things to either directly use the power or encourage the morals to use more of it. Monster spawns were an early favorite, especially since the monsters had useful parts that could be harvested and use for alchemy and crafting.
Dungeons were another innovation that lead to a major leap in power utilization. Originally a safety feature to create mana orbs in areas of dangerously high mana concentration, it was eventually determined that these orbs were themselves quite dangerous if not drained and would continue to absorb mana until they exploded. The Dungeon subsystem was added to encapsulate these cores and drain their power by creating subspaces populated with created monsters, and valueable items to lure people to the Dungeon. If a Dungeon isn't cleared often enough the core becomes dangerously overloaded until it has to dump power by creating and releasing large quantities of monsters - a bad thing for anyone living nearby.
Seeing how Dungeons worked so well, the System Lords decided to extend some aspects of it to the rest of the world. Specifically, loot drops. Hunting creatures for their meat, hides and other body parts is all well and good, but when you can get actual money and items right off the corpse? Almost overnight loot drops became the most successful addition to the System in terms of motivation. Adventurer's guilds, previously only found near Dungeons, sprang up all over the place. Hordes of adventurers sallied forth in search of the most awesome loot... ok, mostly for drinking money and enough cash to upgrade their gear.
And that's how it is that you can collect money from creatures that have no rational need for it. That lizard you just killed has probably never seen a gold piece in its' life, and the only time it has ever seen a magical weapon is while Harold over there was hitting it with one. That doesn't stop us from picking up a few gold pieces and a crappy magic dagger from it.
Just remember to thank the Gods and the System for their bounty. Maybe they'll appreciate it enough to rig some results in your favor in future. Can't hurt, eh?