This is very wrong you need to update your understanding. USD has lost a lot of value this year, which means that this stablecoin is not really stable in terms of value. The price changes all the time. Obviously the price of 1 USDT is not going to change in the USDT/USD pair. 
Try viewing it as USDT/other fiat and you will see. They are both unstable and volatile.
Try viewing it as USDT/other fiat and you will see. They are both unstable and volatile. People don't care if the USD has lost its value. They only want stablecoins to help protect themselves against the volatility of traditional cryptocurrencies. And so far, stablecoins do a great job at it. 1 USDT = 1 USDT, right? Same goes for USDC, USDD, TUSD, and the likes.
What can break stablecoin's stability is the issuer's inability to back its value with real USD reserves. Collaterized stablecoins can also lose their peg to the USD if the assets in which they're backed with lose value all of a sudden. Nothing is 100% guaranteed to be safe. Not even the money in your bank account. Just diversify your investment, and forget about the rest.