You cannot transfer USD from Poloniex to Coinbase.
And Poloniex does not have a USD account. USD would be a fiat account, whereas Poloniex stores USDT, which is a cryptocurrency account that acts like USD, but it is not USD.
To transfer your funds from Poloniex to Coinbase, do the following:
In Poloniex, use your USDT to buy Bitcoin (BTC), Ethereum (ETH), or Litecoin (LTC)
Transfer your BTC/ETH/LTC from Poloniex to Coinbase
In Coinbase, you can sell your BTC/ETH/LTC to USD
Of the 3 funds, I would probably recommend converting your USDT to ETH because transferring Ethereum from Poloniex to Coinbase will most likely be the fastest transfer and will incur the least amount required for the transaction gas fee.
The reason you have to transfer crypto from Poloniex to Coinbase is because Poloniex is crypto only. No fiat.
If you have any questions, let me know. Hope that helps.