Richard McElreath has some nice points about variable independence assumptions. Concretely, for a model this assumption represents the state of "ignorance" and

Richard McElreath has some nice points about variable independence assumptions. Concretely, for a model this assumption represents the state of "ignorance" and is the most conservative choice (i.e. doesn't assume any correlation). That said, he also mentions the "mind projection fallacy" of confusing epistemological claims with ontological claims, which may in turn be the reason why many assume normal distributions for everything.