Because USDC and USDT are both cryptocurrencies where USDT is a stable coin which aims to remain 1:1 with USD and issues by Tether Limited. USDC is another stable coin which if I am not are offered by coinbase.

The difference between USD and any stable coin is that while USD is official currency of USA but these stable coins are meant to stay relevant and stable in terms of USD.

Now coming back to why some exchanges do not offer USD markets? That is because they might not have the right means of supporting it like users are not allowed to withdraw and deposit USD so it does not make sense to allow a currency which user cannot withdraw. USDT markets are same though and serves the same purpose. Binance though offers USD/BTC market (I believe for US members) as you may visit here - https://www.binance.us/en/trade/BTC_USD