The interest they give to every loan is sometimes actually high that it really gives them a big time profit and earning. Imagine not paying in time would give you an extra payment? That's how they generate money plus I don't know if some bank literally have connections in each other because if you use other bank's atm it will cost you too much fee since the bank you have is not available in some places so you have no choices but to use other bank that will cost you too much transactions fee from my experience. This is centralized work that's why most of the people prefer to use decentralized system.
I think banks does this on purpose so they can benefit from the extra charges they get for using other ATM that isn't theirs. Sometimes the banks will claim they don't have money in their ATM so you can make use of other banks ATM then they charge you for that transaction. Banks has multiple ways of making money from their customers. They can also make money through maintenance chargers.
Some banks charge when you make deposits and also withdrawals, the banks are scamming us and that's why bitcoin is better because we don't have all this changers expect miners fees but it's not as much as bank charges unless when the network is congested.