Using a bank as a poor person is stupid expensive. Have less than 200$ in an account? That's a monthly fee. Use your debit card less than 15 times last month? That's a fee. Need checks for that account with only $100 in it? Most people get them for free but you pay. For a time, TD Bank was even applying transactions to your account out of order so you would incur more than expected overdrafts and accrue more overdraft fees than you actually should have had. They lost a court case about that and had to pay people a few dollars, because class action suits are a travesty of justice often.
With a $15/month charge for having less than $1000 in the account per month.
With a destroyed credit history when Wells Fargo and BoA continue their stock value pumping shenanigans and opening accounts in your name.
Edited to add: Wealthy privileged HN readers would not know this, but poor people who are "unbanked" have to use USPS money orders to pay bills, and the USPS charges a fraction of one percent on money orders. So yes, the USPS is already performing banking functions.