I have a family member who works at a bank and has been having to field some of these situations. This is all anecdotal, but at least should help fill out some of the possible situations.
+ A fair amount of people have savings accounts and nothing else. They don't have a debit card and don't use that account with any regularity.
+ There are also some folks who have checking accounts but don't have or don't want to use debit cards. I don't fully understand the why, it sounds like this mostly older folks who don't trust them.
+ There are some access issues with banks. Most banks still want you to be in person for opening an account, and you need to be in person if you want to get cash. For some folks who aren't very mobile, most local banks don't have enough branches.
+ There are some people who really struggle with the paperwork. It's not a difficult form especially for the bank in particular I'm using a reference and the bank tellers will help you fill the form out (it's a page with your name and address, and your social security number basically) but that's enough of a hurdle it precludes some people. Some of the information that's collected here is required by the feds to limit moneylaundering, it's not just the bank.
I suspect it happens but have not heard of any cases of fees being directly the cause. I know I've had a few conversations where someone was surprised to find out you can do banking without fees, didn't understand it was possible to ask for a rate schedule, didn't understand ATM networks, etc. I suspect there's a financial literacy gap that's a cause here (and many subpar banks being fee happy).