Why is lactase not normally produced in mammals?

Lactase is an enzyme produced to digest lactose, the sugar found in milk. In most mammals, lactase production declines after weaning, as the dietary need for lactose diminishes with the transition to solid foods. This reduction is a result of evolutionary adaptations, as continued lactase production is not necessary for survival once milk consumption stops. However, some human populations have developed lactase persistence due to genetic mutations, allowing them to digest lactose into adulthood.