Why would it be so unexpected? An entire village wouldn’t have a coin between them?
Most of the time, coin of the realm wasn’t used for transactions, all was barter. And gold coin was scarce. After the Roman Empire passed its financial peak (reign of Trajan who had a nice big payday due to his conquest of Dacia; Trajan ruled a couple reigns before Antoninus Pius) the Empire’s population continued to grow, and more and more demand for coinage took place.
The response was bronze coinage, which had taken over nearly entirely before the traditional fall of Rome. The Roman bronze coins tend to be in very bad shape if they’ve been in the ground (and typically, that’s where they are found), but currency is merely a medium of exchange — a fact identified by the Romans.
There was a revival of gold coin minting during the peak of the Byzantine phase of the empire, because Constantinople got fabulously rich from trade and taxation. Gold Byzantine coins at least used to be pretty reasonably priced, mainly because gold survives well, and they were minted in pretty large quantities.
There’s a misconception that there’s enough gold around to mint into usable coin; the entire pile of gold ever mined in history wouldn’t make a cube 1000 inches on a side. 1000 cubed is one billion (cubic inches), and the population of the Earth today is between six and seven billion people.