In addition to the other administrative burden related explanations, there's at least some fraction that are cultural.
Ages ago, I read a "Nickel and Dimed" style long form article about check cashing services where the author tried to understand your very question: why do some customers continue to use these services when cheaper alternatives exist?
IIRC, some preferred the accessible, high touch in person aspects of the services. Much like some people connect with their regular barista, I suppose. Vs the intimating experience of a bank, the impersonal ATM, whatever.
In that way, putting on my UX and ethnographer hat, I keep thinking of the long tail of late adopters, as first detailed in Diffusion of Innovations. People are just people and have all sorts of nonobvious reasons and factors guiding their choices. We just have to meet them where they're at. Especially with social policy.
Any way, it's important to ask why people don't take advantage of apparent benefits, like you've done, and then figure out catalysts for making adoption more likely.
Edit: Oops, I forgot my two concluding points.
For accessibility, sociability aspects, I think the USPS branches will be great for community banking. Every branch I've ever visited is like its own community hub.
It's not enough to ask people why. A lot of the time, people can't articulate their rationale. This is normal. Just like with usability testing, would-be policy makers have to actually be present and observe.