The 1:1 peg that ‘stable coins’ like USDT/USDC claim is a promise and not a guarantee.
What currently keeps the USDT/USDC and all other ‘stable coins’ from going to zero is that the number of people entering the crypto-market is much bigger than the number of people cashing out back to fiat or exiting the crypto-market.
If even a small fraction of people decide to cash out, the ‘stable coin’ issuers will stop serving requests in order to preserve their fiat holdings.