Mammals drink boob juice (it’s in the name)

Mammals grow up and stop drinking it, so they stop producing the enzyme.

If you don’t stop (i.e drink a cow’s milk after breastfeeding) then you keep producing it I’m pretty sure I read about continued ingestion can re-enable the enzyme production (as in, you can eat yourself tolerant again)

There may be genetic conditions that make the intolerance permanent though IANAD