An entity with a single purpose (accounting to issue no more of X tokens than they have Y tokens in another account) has a vastly easier challenge (both directly, and from audit/controls perspective) than a more-powerful entity with lots of conflicting needs. There are still incentives for people at a custodial stablecoin to steal/debase, but it is a relatively tractable problem compared to e.g. federal reserve dual mandate or a non-independent central bank with even more competing concerns.