Is it a "structural" issue? Or is it just the fact that, statistically, poor people are really unlikely to pay back loans, so loaning to them profitably means charging really high interest? Or is that a "structural" issue?

Bluebird (mentioned by the parent) is a checking account alternative (read: not a real, FDIC-insured bank account). No one's loaning anything in this case. It's designed to appeal to people who, for one reason or another, cannot afford to open even a basic traditional checking account.

Edit: Before such ersatz-bank-account services proliferated, I believe it was common for poor people to cash their paychecks at a check cashing place (which costs time and money).

The profit model is to fleece the most reliable customers.