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!!
Those figures are without a doubt very high for normal people like us, but for a business like that it's to be expected that they get those kind of profits, I have always thought that the fastest way to make money is to go directly where the money is and there is no business that gets closer to that than banking and exchanges, have you seen how powerful are the banks? This is one of the reasons of why satoshi created bitcoin, and while we do not have bitcoin banks we have exchanges which are becoming big enough that by themselves they can alter the price if they want.