I assume that they're talking about not having the minimum funds available to open an account or not being able to maintain the minimum balance and having the fees for not doing so being significant.

https://www.forbes.com/advisor/banking/bank-account-minimum-deposit-minimum-balance-requirements/

A minimum opening deposit is a certain amount of money—usually $25 to $100—that a bank or credit union requires you to deposit to open a checking or savings account. Some financial institutions do not require a minimum opening deposit.

https://www.thebalancemoney.com/typical-minimum-balance-for-traditional-savings-accounts-5204547

When you open a traditional savings account, you may need to keep a certain amount of money in the account to avoid a monthly service fee. The minimum balance ranges from $300 to $500 in most cases, while the fees typically are around $5.

The bank is going to make their money by lending out the money you deposit, so they're going to want to have some money kept in the account.

Might also be that "unbanked" is a euphemism for illegal immigrants who don't have bank accounts because they're worried about showing up to immigration enforcement or something. It looks like, while an illegal immigrant won't have a Social Security number, there are ways to get around that.

googles

Or maybe that they immigrated legally from somewhere where banks aren't a very safe place to put money.

https://www.cnbc.com/2021/10/12/more-than-12percent-of-hispanic-households-are-unbanked-why-that-can-cost-a-lot.html

More than 12% of Hispanic households don’t have a bank account.

Still, Hispanic households are less inclined than white, non-Hispanic ones to use the banking system. In 2019, 12.2% of Hispanic households were unbanked, compared to 2.5% of white ones, according to the FDIC’s latest survey. Black households had a rate of 13.8%.

The reasons for staying away from bank accounts may vary. There may be language barriers or a distrust of banking institutions, especially for those who may have come from a country that had a weak or corrupt system, Espinal said.

The main reason people are unbanked is the belief that minimum balances are too high for them to meet, said Karyen Chu, chief of banking research at the FDIC. Therefore, they will incur fees.

I mean, a mobile phone is going to cost something too, but I think that that's kinda considered to be an essential by a lot of people today. Like, they're going to want to have a phone to have a phone, independently of how they might use that phone to spend money.