Why do banks, whose fundamental job is to accept money and then return it to the customer on demand, struggle to meet the "giving the money back to you" obligat

Why do banks, whose fundamental job is to accept money and then return it to the customer on demand, struggle to meet the "giving the money back to you" obligation. Why isn't there be a bank that doesn't fuck around with my money, so I can always be guaranteed to be able to get it out whenever I want. I'd gladly pay $20/mo or so for such a service. Another case of: "If you aren't paying for it, you are the product." Earning US$500 taxable annual interest on an account with balances averaging US$100,000 isn't particularly valuable to me, especially if the edge cases are that the money I gave them might sometimes not be available to be returned to me when I need it.