For whom? The least volatile asset relative to dollars are dollars. The moment you start building a money-market fund you have novel risks, from credit to rates and FX. (And, in the extreme, legal.)
For people outside of the US.
They’re better off with their own currency or an international currency (currently, Euros or dollars). Almost never a basket. (The IMF has SDRs. And various trade-weighted baskets are constructed for metric purposes. They aren’t practical instruments.)
Which makes it garbage for transacting if you’re paid in or pay with any other currency.
Modern currencies split their store of value and medium of exchange components. Stablecoins (and credit cards and checking accounts) seize on the latter. Bitcoins (and Treasuries and money-market funds) the former.
It's about making free money, not about making sense. They back it with USD because recent US regulations allow stablecoins to be backed by US securities. They will buy these securities, and for every dollar converted into their coin, they will earn a percentage per year. Free money.
The Australian dollar has gone up about 20% vs the American dollar since Trump.