actually exchanges that allow fiat <-> crypto conversion such as Kraken and Bitstamp offer several crypto "bridges" aside from Bitcoin: usually Ethereum, Litecoin, Bitcoin Cash. Bitstamp also offers Ripple to EUR and Ripple to USD conversion. The key here is that Bitstamp itself runs a node of each of the BTC, BCH, LTC, ETH, XRP blockchains and can thus have a view, at every moment, of all the balances of these cryptos. So if you want STEEM <-> USD or STEEM <-> EUR conversion you need to persuade Bitstamp to operate a node on the Steem blockchain