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.
Because it's easy to use and actually have USD in their exchange you can trade your USDT to USD here
https://www.binance.us/en/trade/USDT_USD.
The reason why many exchange doesn't use direct USD conversation is because it will make it difficult for them to have , if you want an easier conversion to USD you should find an exchange that accepted in US like coinbase.