I don't think you will find a satisfying answer. But some random jumbled thoughts that come to my mind: Among the Dyirbal traditionally there was a strong avo

I don't think you will find a satisfying answer. But some random jumbled thoughts that come to my mind: Among the Dyirbal traditionally there was a strong avoidance taboo with certain relatives: > The overarching feature of all Australian Aboriginal societies is the classificatory kinship system. By applying a series of principles, each person is in a relationship to every other person. Mother’s sisters’ and father’s brothers’ children (‘parallel cousins’) are regarded as equivalent to one’s own siblings. In contrast, ‘cross-cousins’, the children of father’s sisters and mother’s brothers, have a quite different status. They must never be directly addressed and, if talking in their presence, a special speech style called Jalnguy has to be employed. [1] Lizards are an important source of protein for the indigenous Australians. Verbs of motion were similarly taboo among the Dyirbal, just as specifying a specific type of lizard in front of your MIL. In most cultures, the mother of a possible wife has a significant say over whether the marriage will happen. In many cultures, a demonstration or contribution of wealth or ability to provide may be required of one or both potential spouses. In nomadic hunter-gathering cultures, men's social status is often tightly linked to hunting prowess. And indeed: > [...] until the boy had proved himself to be a good provider. When he brought home a carcass, the choicest portion would be ostentatiously presented to waymin, the potential mother-in-law, as a hint that she might soon consider him to be ready for the marriage. [1] I will now speculate wildly. If you're looking for a why, assuming there might be one, look at the effect. It leaves the man unable to talk about his hunting activities in front of his in-laws. He can't easily brag. He can't easily plan a hunting trip with his father-in-law. Preventing such close cross-family links outside of ritual interactions with a go-between, may prevent a single family from gaining disproportionate influence or power. It may also help preserve the expected degree of distance between the son and mother-in-law. A (potential) son breaks many social taboos by discussing his ability to provide for his (future) wife or if he tries to coordinate economically with her family, outside of the appropriate channels. [1] p. 5; R. M. W Dixon; Edible Gender, Mother-in-Law Style, and Other Grammatical Wonders: Studies in Dyirbal, Yidiñ, and Warrgamay https://academic.oup.com/book/27709 https://academic.oup.com/book/27709