If everyone makes their own currency like you said, then each one of those currencies will be worthless. Who's going to want to take your currency when they have their own?
then there will be no trade, if everyone has their own currency. because all money will be of no value and the business world will be destroyed. then events like this must be prevented so that the world will not be damaged. hence the creation of a currency is given certain limits and conditions and not every individual can make it freely.