Because banking is sticky. People are more reluctant to change banks than cell phone providers. Consider a big bank like BofA with $2T in deposits. Let’s assume 1/3 is just going to be your average Joe or Joe’s plumbing with 10-20k in checking accounts and another 20k in an emergency savings account. Those accounts are yielding 0.02% currently and have been (or less) for a while. So the price to service these accounts is 130M annually. If they were to just automatically pay a competitive 3-4%, that 130M goes to $20B. They know they are too big to fail and if they experience a run the fed will are in, which they have already. Large banks have literally no reason to pass along a good interest rate to customers bc, frankly, they don’t want their deposits.