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How did humans evolve lactose intolerance (lose the ability to digest lactose in adulthood)?
I am not referring to how certain Eurasians, Africans, etc developed lactose tolerance in adulthood thousands of years ago.

Instead, I am talking about how virtually all human babies are born lactose tolerant with the ability to product a lot of lactase enzymes to be able to drink and digest a lot of milk....but then a gene kicks in to turn off the lactase producing enzyme when they grow into older children/adults. Thus, I'm asking how and why the gene/traits/etc to turn off the lactase producing enzyme in adulthood evolved to create lactose intolerant adults when the default seems to be everyone starting out from a point of lactose tolerant babies.

I've tried to research this myself, but everything I've found talks about how humans developed lactose tolerance in adulthood rather than how babies lost lactose tolerance when transitioning into adulthood.