Even if you live out in the middle of nowhere, you have to pay property taxes. If you barter, you have to pay taxes on that stuff too, so you need to make the fiat bucks somehow. That's why fiat being required for taxes is the big one that makes it the defacto standard. After all, you have to have it, or its electronic equivalent or people with guns will come and get you, imprison you and take all your stuff away.

Not that paying taxes is bad. It helps keep inflation down vs the government just borrowing all the fiat it needs from the fed. However, if you try and run your whole life in gold or bitcoin or whatever you just can't do it legally.