Nothing stops people from going to the beach and collecting shells. Except that takes work. You’re out there collecting shells instead of farming or hunting. And the beaches aren’t just covered with cowrie snails, you have to look for them.
Shell money started out simply as a luxury good. The shells are pretty and can be made into jewelry. But they also have properties that make them useful as a key good for barter…they are durable, small, fungible, divisible, transportable. So while before people would gather the shells occasionally to make pretty jewelry, they moved on to gathering the shells just for their value as a key good.
It’s just like a system where cacao beans are a key good. What’s to stop you from becoming a cacao farmer, growing millions of cacao beans, and becoming rich? Or becoming a distiller and manufacturing rum and becoming rich? Or a tobacco farmer who makes cigarettes and therefore becomes rich? These key goods are goods that have actual value, or at least are percieved to have actual value. But the manufacturers don’t become millionaires simply because they produce the key good, any more than owning a gold mine automatically makes you a millionaire in a country that uses gold coins for money. The key goods are still goods that are subject to the laws of supply and demand (and so is paper money, if you think about it, which is why inflation can occur).
So if the beaches are covered with shells, and it only takes a few minutes to gather a fortune in shells, those shells would never have become a key good or a system of proto-money. Instead it takes work to gather the shells, which is a forgone opportunity to do other work. So the value of the shells hovers around some equilibrium depending on how easy the shells are to gather, how long an average shell lasts before getting broken, how often shells are traded, the size of the economy relative to the supply of shells, and so on. Just like money based on gold coins depends on how much gold is mined, how much gold is lost, or traded overseas, the demand for money, and so on. And so we have situations like the Spanish conquest of the Americas that flooded Europe with gold and silver and made Spain incredibly wealthy and powerful, but also created tremendous inflation and destroyed Spain’s domestic industry and eventually drove Spain to bankruptcy…because gold can be used as money but it doesn’t actually create any new wealth.
So discovering vast new beaches full of cowrie shells could make the discoverer rich in a system with shell money, but the net result will be inflation and a sudden decrease in the value of each shell. Find enough of these new beaches and the system of shell money collapses because shells are worth too little, and the guy with millions of shells who thought he was rich finds out he’s ruined.