Of course they are not, but UST is. Stablecoins are different from each other. USDT, USDC and DAI are all collateralized (at least partially). UST is minted out of void by burning LUNA, creating demand for LUNA so creators get a profit and turn away, slashing all UST investors. It is an outright ponzi scheme hidden behind layers of DeFi jargons and borrowing trust from true stablecoins like USDC while being fundamentally different from any other responsible stablecoin.