Why not rationalize?

Everyone else does, now including capuchin monkeys. Rationalization of individual choices is not, I think, the interesting case, nor the source need of the behavior. Rationalization is essential to belonging to the in-group, so all social animals with significantly complex brains need it, since they gain great survival advantages by membership.

Take a look at human society. There's no end of truly weird things people can believe in order to belong. There are even a few people who still believe in Duhbya.