The thing is, all the exchanges with a large number of Altcoins work with USDT, and dollar is not in use there.
Aside from Bitfinex, which had issues with the withdrawal of the dollar for a long time, all the other exchanges work with USDT,
Binance, Poloniex, Bitrex... Bitstamp is a good exchange to work with dollar, it operates with large volumes of trades, but there are few crypto-coins in turnover there.
the reason not many exchanges deal with US$ fiat because they need to follow local regulations
they need it to be legally do fiat exchanges and open real banking account
and following the rules, they need to comply KYC AML stuff required as a registered money transfer service
btw as far as I know Poloniex doesn't have 'dollar' (US$) fiat markets