In the last statistics I can find is from 2019, there were 7 million US households do not have a bank account. So that is over 18 million people (2.6 per household) without a checking account out 328 million or 5% of the population in 2019.
The reason are believed to be:
- not enough money
- high unpredictable fees
- distrust of banks
Their checks are typically cashed by check cashing and payday loan facilities, at unusually high costs. This is also where they pay bills with no checks as well for a high fee.