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.
does binance not accept crypto purchases using USD? Since late 2019, binance has enabled buying crypto using USD with debit or credit cards.