One reason people use cash is that you need ID to open an account with a US bank. You can't just walk in with cash to put in the account, and get a passbook or debit card or checkbook. (Or you could have a bank account you opened before 2001. Or if somebody with an existing account trusts you, they're allowed to turn their individual account into a joint account with you and nobody asks for your ID.)

Undocumented=unbanked, and a LOT of people are undocumented. Sometimes because of immigration difficulties, sometimes because they left their documents behind when fleeing abuse or flood or fire, and haven't caught up with the paperwork. Or because they became adults during the pandemic, and could not cope with government offices that slowed to a socially-distanced crawl.