Second of all, especially in smaller communities where this practice is more likely to occur, there is a social capital gained by doing it which is not necessarily directly monetary, but I think it's pretty easy to see the rational self interest argument for "being part of a community" outside of just purely monetary gain.
> It is very rational to lend money with zero risk if that gets you more business.
Well it's definitely not zero risk, and when credit cards exist, which deal with all the risk, it's not rational to lend money without credit cards.