Stablecoins are a necessary legal hack.

The US has regulated itself into a corner when it comes to AML/KYC. Those regulations ended up causing more problems than they solve, but they can't ever be undone. If something ever happens, like a terrorist attack funded by money laundering activity that the existing regulations could possibly have prevented, the blame will fall squarely on the shoulders of the politicians who decided to undo them.

It's much easier (and politically safer) to say that stablecoins are just a different asset class, and hence very different regulations should apply to them. This essentially lets politicians design a parallel, much more permissive financial regulatory system from scratch, with many lessons learned from the existing one. If something ever happens, it can always be blamed on "those pesky stablecoin issuers who keep prioritizing profits over the security of our nation."

From a purely technical perspective, any stablecoin could be replaced by a centralized database mapping public keys to balances, at much lower cost and with very little loss in functionality. That, however, would look too much like a bank from the regulatory side.