Thanks for your reply.
Why do you think that some brokers does not have BTC/USD? For example, Binance. They have BTC/USDT and BTC/USDC but not BTC/USD.
Trading with real USD rises more problems which exchange does not want to have. Therefore, Tether is created to solve the problem. One USDT is equal to 1 USD and it is secured by Bitfinex. Dont worry much about this issue cause it wont affect your trading styles.