Why can noun class membership not usually be predicted from a noun’s meaning?

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Thematic map of Africa showing the distribution of Niger-Congo language branches. Distinct colors represent language groups such as Atlantic, Mande, Gur, Kwa, Benue-Congo, and Bantu, with Bantu languages covering most of central, eastern, and southern Africa. A numbered key lists prominent languages in each area, and country borders are marked for reference. — Niger-Congo languages Map showing the branches and distribution of the Niger-Congo languages.

In the Niger-Congo languages, noun class membership cannot be predicted by a noun's meaning because the original semantic categories that once organized these classes have broken down over time.

While it is highly probable that these systems were originally based on semantic groupings—such as assigning all humans, animals, body parts, or liquids to specific classes—these clear-cut categories no longer function as reliable indicators for class assignment. Consequently, nouns must now be assigned to classes based on their specific singular and plural forms rather than their definitions.1