Because you need some way to inject new coins into the system initially. Bitcoin chose to do this by allowing early users to "mine" the currency.

The traditional approach is to have a central bank which does the minting and printing, and then the regular banks can borrow that money. Those banks then lend that money to their customers, possibly creating more virtual currency by fractional reserve banking.

Bitcoin obviously can't do that because it is a decentralized system. The traditional approach also has the disadvantage that all currency in circulation is ultimately borrowed, so there is always more debt than money unless you allow banks to default. That can only work as long as the economy is growing fast enough.

I think there are even better alternatives, by modeling currency after trust and social capital: http://ripple-project.org/decentralizedcurrency.pdf