If Americans can't afford a $400 emergency, then they can't afford the risk of switching banks. In paycheck to paycheck living, the large bank is both a bedrock of an otherwise unstable living condition and an Overlord you dare not cross.
My bank is also where I also get home, auto, and life insurance. I actually go out of my way and get auto loans from them even when the dealer offers lower interest rates. Because the bank has been good to me over a 30 year period. Yes, I have been a customer that long.
Given the mediocre or non existent interest rates of the last 10+ years, why is anybody keeping lots of cash around unless it’s rainy day funds or parked awaiting market improvement? One has to beat inflation to get anywhere and bank interest isn’t going to do it.