Because 6x3 foot iron and bronze ingots and shiploads of wheat are hard to move and difficult to store. Which makes always swapping one for the other impractical. Requiring a medium of exchange and the ability to track who’s got what and where they’re sending it.
Money has value because it can be exchanged for goods and labor.
Discounting gold bug fantasies about money before so and so ruined it being rooted in things with intrinsic value, this is how it works. I can get stuff for money, I can store money instead of stuff and in different ways than stuff. And I can get paid for work in money instead of stuff. Those magic substances with intrinsic value usually aren’t. Like gold, which doesn’t really have much intrinsic value, or practical use. Wasn’t the original currency, and wasn’t always the go to currency. Salt, Iron, and before the development of cash just ledgers were incredibly common.
(ETA: Edited the above for clarity.)
What “backs” modern fiat currency (which isn’t a new concept at all) is it’s utility. That utility is ensured by a whole host of complex things, primarily rooted in the stability and total economic output of the entity that issues it.
At it’s simplest. A big, productive, stable country represents a big chunk of people and business entities who are willing and obligated to accept that currency. Which means it has utility now, and for the foreseeable future. Boom. Money has value.
Ultimately speaking it works, and does it really matter if you can eat money if it works?
This is all very well understood, the history is very well documented.
Much of this particular discussion boils down to stoner level “what if the apple isn’t real” deep thought combined with scaremongering and speculation about what will happen if or how it might stop working. With lack of some sort of alternative medium of exchange pegged as the big problem.
People often forget how many currencies, and how many nations failed and collapsed during the era of the gold standard. As if the Great Depression didn’t happen in the 30’s, and we didn’t move off it to help stablize things. None the less looking any further back in history.