Silly, you can find that on google, as its not a secret how legal tender works:
Dollars are now backed by the U.S. government itself . As legal tender, the dollar is accepted for both public and private debts. The dollar’s value fluctuates with economic conditions and the federal government’s management of interest rates.
So they can set value to it, by printing more of it, or by interest rates, they can skim the surplus from economy by the power granted them by the PEOPLE. And PEOPLE can change it. Their faith lies in themselves, the choice they can make.
By how changable fiat is, power lies within it. Democracy. Of course you can change it to react to needs. But it needs correct treatment.
There are mechanism to make it stable, that are not available to BTC! BTC is not democratic, its algorithm you can compare in fact to a dictator with a small weewee (limited amount of tokens), and a big ego (look how powerful I am, just you wait!). Dictator that will stay itself on the same childish level of understanding the economy no matter how much you want him to GROW. It cant GROW. BTC will stay BTC. Its nature doesnt change, its only PEOPLE that can accept its flaws and profitability. Its like a deal with dictator who is completely unstable.
You can also see it being completely stuck in place it is now. Because there are reasons why it cant spread further than today, and there are reasons why it will still be treated as a speculative resource. Will YOU distribute your bitcoin to citizens of your country, even when they dont want it? Or maybe YOU will want your government to take bitcoin from YOU and distribute it?
Magical thinking if you ask me, that it will magically just spread. YOU have to do the spreading. You would have to distribute is as much as you can, now. Buy everything you need with bitcoin only. NOW. YOU will spread it. Without guarantee that it will ever be stable like dollar.
And to think that alone would make it stable, when you will see the economy being a live thing that evolves and forcing BTC into currency role is going to solve what exactly? Just buy bitcoin and leave fiat also? If fiat is still more stable, what will be the allure of BTC? Its just volatility that is alluring with it, and it will be kept volatile, because this way it can be more PROFITABLE.
Right now, will you wait like Bukele for it to wage on the decision of banks to give you loans in fiat? Because nobody wants to credit a gambler. Who will give him loans? You? Will you just give him all your bitcoin for it to take off? Or you just want to keep bitcoin so you want to have more stuff later? To MAKE IT.