fiat currencies at least have the value of being a generally accepted item with an exchange value that is easy to use to trade for things, and its backing is usually the existence of the states that made them and the institutions and laws they use to back things. bitcoin/ethereum/etc are mostly useful as a speculative commodity. in the terms of capital, they are much more M-C-M (exchanging money for a commodity, at the hope of selling it later for more money) than C-M-C (producing something to get something else that you need).

if tether, for example, was operating by minting regular currency rather than cryptocurrency, it would be so obviously a giant scam. creating fake dollars to buy other fake dollars to bump up their price to back the fake dollars they made and repeat.