There are a couple of main reasons (although many more) why most countries have their own currency:

1. Each country wants to be able to regulate their own economy, having their own currency that they control gives them that power.

2. It provides a small barrier between a nation's economy and the global economy. An example of this is Greece with the Euro, though luckily it was contained to Greece as far as I know. If a nation's economy collapses and they have a worldwide currency it could be very bad.