Banks in the US are tricky business sometimes. They often want a lot of info (understandably) but someone does not have a home may not be able to provide some of it. They may not have a phone number, they may not have an address, they may not have either. Could also be undocumented individuals are part of it, I'm sure they factor in. Maybe someone decided to go "off grid" and stop using anything connected to the government they can and is now a mattress stuffer lol. Point is cash should always be acceptable outside of edge cases IMO.

Or if they are poor, is there any kind of benefit/welfare from the government?

Yes but it's very mediocre generally unless their state/city has invested more into social programs, which is the exception and not the rule in the US. In my state unemployment insurance (welfare) is I think...237.00/wk so not even $1,000/mo (haven't been on it in a few years now forgot exact number).