One, because the consequences of an AML failure are immediately catastrophic: a dollar stablecoin can't reasonably survive without access to the dollar financial system. Tether's unregulated status means enforcement options are zero or ban. (Banks can be fined and put under compliance regimes.) Running a perfect AML programme indefinitely is impossible. There is no indication Tether runs much of an AML programme at all.
Two, because bank-like structures are inherently unstable. Asset values fluctuate. The banks they hold cash or assets at will fail, sometimes beyond deposit insurance limits. Unlike banks, Tether has the benefit of being able to block redemptions. But they can only do that so many times before a regulator takes notice.
In summary, Tether exists at the pleasure of American regulators and the consequences of losing favour are collapse. They're compliance-wise and financially unstable. The consequences of a single failure won't be catastrophic. But eventually they will fail for the same reason every bank without deposit insurance will, eventually, fail.
