I think they are acting as fillers because over 50% of all minor Natives are based on USA and Canada, which have several maps.
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South America is diverse but there are only a handful of minor civs restricted to a few maps.
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Brazil alone is 85% of the size of USA, however: Compare their maps and tribes.
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Asia is huge and there are only five minor natives. So they fill everything with Jesuit missions in order to have more than one settlement per map.
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We give a pass to Africa because only the northern part has been explored so far.