Because everybody else does. The Bandwagon effect. And because you have to use it (most people).

Fiat is the only accepted way to pay bills, parking tickets, mortgages, car payments, taxes, etc. This is also most people's only source of income - fiat.  This is all assuming you actually do any of the things previously mentioned.



And more simply stated by my friend's previous post, "because you can buy more shit with USD than bitcoins, common sense."