Some know don't have a bank account because they simply don't trust banks. They want to have access to 100% of their money 100% of the time.
Some don't have a bank account because they don't have income (adult dependents)
One didn't have a bank account because she was a minor and there was no adult around willing or able to open one for her to use.
When I was a kid I knew of a few adults who had bank accounts closed on them for check kiting, including my parents - I know of this because I overheard people talking about it quite a lot. I don't know what ended up happening after that, if they were able to open a new account or what. I know you probably wouldn't be able to open a new account nowadays with Chexsystem and the like. Of course, check kiting also isn't a thing anymore either.