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, even if we're not doing the math here, everyone knows how crazy it is. But, "With great power comes big responsibility". Exchanges may not earn as huge profit as that if there are a lot of issues in your business. Consistency, Response, and Stability these things must be present in a certain exchange.