Customs are bad for that. Folk explanations of why people do something take on a life of their own. Most clearly, perhaps, in the way that the primordial, pagan origins invented by Victorian folklorists have been passed around for things that can be proven to be modern in origin.
But in fiction, you're more constrained. Readers put more credence in what the characters say. Partly because writers use it as a form of info-dumping, and they know it; partly because the plot of hunting about trying to figure out what is going on is not the sum of all plots, and they will accept all sorts of conventions to get swiftly through it and into the meat of other plots.
It's a trick and a half to try to get readers to give them no more credence than such account deserve.