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Why do permanent molars not replace any primary teeth?

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A labeled cross section illustration of an adult human molar shows the tooth's crown and root, with layers including enamel, dentin, and pulp, as well as gum, cementum, blood vessels, periodontal ligament, lateral canals, and nerve. — tooth Cross section of an adult human molar.

Permanent molars do not replace any primary teeth because they erupt in the spaces behind the primary dentition as the jaw grows and expands.12

While the primary molars are replaced by permanent premolars (bicuspids), the 12 permanent molars emerge into the mouth cavity independently of the primary set. This process allows the total number of teeth to increase from the 20 primary teeth to the 32 teeth found in the complete adult dentition.21