The yard sale model doesn't attempt to model an actual economy. It's a thought experiment to counter causality bias.
Human minds tend to be biased towards causal explanations. So if we see huge wealth disparities, we're biased towards thinking that these disparities must exist for some deeper reason (often argued to be meritocracy). The model counters that thinking, by showing that, even in a very simple model with rules that seem fair to everyone, huge disparities can appear entirely at random.
It doesn't prove that the disparities we see in the real world are fully random. It invites us to question the assumption that they aren't.
Human minds tend to be biased towards causal explanations. So if we see huge wealth disparities, we're biased towards thinking that these disparities must exist for some deeper reason (often argued to be meritocracy). The model counters that thinking, by showing that, even in a very simple model with rules that seem fair to everyone, huge disparities can appear entirely at random.
It doesn't prove that the disparities we see in the real world are fully random. It invites us to question the assumption that they aren't.