Earlier quoted context omitted.
Because the backing of a state provides a veneer of credibility to the currency that your own say-so doesn't. Like, barter economies built on trust still exist and if that's what you want, knock yourself out. But at some point you want to pin down how many chickens your bedroom dresser is worth, which means you need a token to represent a value, which means you need someone or something to provide backing for the tru…
States actually have a very hard time giving credibility to their currency. You see, the problem is that governments are too powerful: they can not make promises that they can't break. In contrast, private companies have it much easier. They can use plain old contracts to bind themselves. Eg Amazon could issue physical notes and promise to redeem then in eg USD on demand. (Wal-Mart could issue tokens they promise to…
In some bizarre ancap reality where there are multi-trillion dollar rug pulls because there's no stock regulation and every company uses their stock to leverage themselves into an unregulated fractional reserve bank and we also have some other third party non-governmental market maker scrip converting institution that tells you how many BezosBucks to a YahooYuan, then yes, I guess you have a world where no state is backing the currency. I would argue that the institutions that govern this process would still become quasi-sovereign.
I don't really accept the Scottish retail bank note system as being non-state backed. For at least the last 200 years there has been some kind of regulatory linkage between the apparent retail bank note and the backing of a sovereign state. I can't speak to the deeper history here, but whether or not there exists song legal- or regulatory- jiu-jitsu, at some point somewhere down the stack a government is guaranteeing it.