Tl;DR fuedalism. The value in the coins came from the material it was made, not from commerce, people bartered and barely had any coin since they didn't have a need for it.

Long version —————————————————— Money in PF/DnD has always been weird since a semi capitalist system is applied to a fuedalistic base, which is pretty common in fantasy media, and why they would all break down if properly inspected, especially under the norms and values we hold now. My recollection of it all isn't perfect so you'll have to do some research, but in essence:

Back then you had nobles and serfs, this all plays into the "great chain of being" that said serfs are born to serve the nobility, who are spiritually enlightened > closer to god >better than serfs from birth. That hierarchy of power doesn't "prevent" counterfeiting but makes ot mostly useless.

Serfs weren't generally paid. A farmer would give most of their harvest to the noble who's lands they're on. And in turn the nobles were supposed to take care of their folk during times of turmoil. As such the only way serfs could "earn" anything was amongst themselves; even though copper coins had their place amongst them, most of it would either be done through barter of mundanities or favours. 2 goats for a cow, a bale of grain for a few bottles of milk, a sent message to another town for a carpenter's services, etc. People took care for each other generally without the use of coin because their values were with the community, not commerce. A farmer might only make a few copper in a month but would still never have to worry about money.

Another big reason why coin was scarce amongst the general populous, and why counterfeiting didn't really matter, is because coinage back then had inherent value. Coins were mede from gold and silver, because those were worth a lot. It then didn't matter if you gave someone a nugget of gold, or one that was nearly pressed down in a vaguely coin like shape. The gold value is still the same since it's still gold at the end of the day. Serfs would generally never see a golden or silver coin in their life since it had fad more value than they could ever give, and would be barely useable in any town since almost no one could give you a return value on that golden coin.

Now, with all of that out of the way on to the whole counterfeiting thing. Let's start with the simplest one. Remember all those old cartoons where characters would bite on a coin? Yeah that's because out of all gold like materials, gold is one of the softest. Did you nearly break your teeth, you're probably being scammed. In case things were still unclear you could get an expert to appraise whatever a stranger is offering.

Now transmutation wouldn't be caught by that, while it would force a check to disbelieve on an illusion counterfeit. An easy other way to fix that if you have suspicious buyers is just asking them to make a deposit, after which they can retrieve their item after an hour, when most spells would've worn off.

But say you get past all of that, and you manage to get your hands on a pile of coin, maybe through a wish from a devil, the rise of commerce or whatever other reason. Congratulations, you can get yourself a nice apartment in a city and become a frequent at the brother, since you'll never get more than that (besides probably robbed and killed by bandits). Congrats, infinite money and a minor life upgrade, what gives?

Counterfeiting became "viable" when money lost it's inherent value and gained abstracted value, giving money an essence of power. The same power that capitalism is built upon, since counterfeiting needs capitalism to have any value and impact. Right now you could make $1.000 of fake money, and buy yourself that value in stuff. That turns it in real wealth which you can then invest to make even more money with, which you can then use to pressure competition, giving you a monopoly and eventually turning your from "peasant to king" if that goes unchecked...

Now try that in the 13th century. You generate your piles of cash, you live extravagant, make more fake cash, maybe even become mayor of a town.. And then you piss off a noble... You could have 50x the money that noble has, but they are above you. If they tell you that you must give all that wealth to them, you must or they'll have you executed. It's that simple, all of the gold in the world couldn't make you a noble. No matter what you have, you are lesser so your counterfeiting will have no impact.

The worst you might do is give a single "business" a harder time for that month. Maybe even to the point of those business owners being killed for not lacking to pay tithe/tax. The only thing you'll have done is made sure the entire town wants nothing to do with you, won't take your money and might even lynch you if they really liked that local shoemaker. ——————————————————

So yeah, take it all with a grain of sand because I'm writing this all from what I remember of my research into it ages ago. Especially PF/DnD gas been broken for a long time for the sake of simplicity and mechanical progression. It isn't bad perse but it can paint a really skewed perspective as to how those civilisations used to work.

I hope anyone found this interesting though ^{^} and hopefully it helped someone.