This is the answer. Running a bank is expensive, even if from the customer's perspective all the bank is doing is provide a website that shows them a number. But that number is probably the most important single number in their life.

Since people are resistant to paying monthly account fees, banks will have to hide the fees in ways that you won't immediately notice. In a way, customers are deceiving themselves. If they just paid the monthly fees, the cost of their bank account would be highly predictable.