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!!
Yes, cryptocurrency exchanges earn a lot everyday especially those exchanges that have a huge volume of trades, 0.3% trading fee is a small amount for a single trader but if there are lots of people who use their platform and sum all of it, it is sure a huge amount of money, but I agree on what other say here, they also spend a lot of money (for better system security, advertisements, pay their employees, etc) to make a competitive services and for people to choose or trust them with their money. And of course exchanges are still business so they will always find a way to make more money.