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!!
So if you think that is too easy then go and start and crypto exchange and earn money as one or $2B per year. 
It is not eary right you need to see the work behind all those exchange and they also have expenses and only few exchanges can earn these kind of earnings in future many competitors will come into the field so they also have to upgrade to withstand the new rivals.