i just did a simple calculation, about 35k bitcoins are traded on GDAX every day. At its current price of $6900 and a trading fee of 0.3%, the exchange puts about $700,000 in its pocket EACH DAY and it is ONLY for bitcoin. After including all other cryptos, this number can go up to more than $2MM. Since the exchange is active 365 days a year, the exchange's annual revenue can reach almost $1bn!!
Well that is how exchanges live and survive and earn money. But not all exchanges have a high success rate. Exchanges spend huge of amount promoting their website and have paid a number of people to be known and to gather more clients. Without clients exchange will bound to fail. I remember the exchange live.megacrypton, it disappeared from the web since only a few users are trading.
Aside from the huge income, the exchanges also make sure that all coins in their site is moving and so they create transaction flows which sometimes are risky and cause them to loss a significant amount of btc.