European humans are weird.
To most European readers, the inability to consume milk is seen as a defect, similar to allergies. In reality, it is by far the norm for humans outside of European populations, and overwhelmingly the norm for nonhuman mammals.
It's theorized that lactase persistence is carried over from Eurasian nomads, from which most Europeans are descended. As an adaptation, it allows populations to consume large quantities of dairy, a renewable food source, while maintaining good health.
Aliens studying this would fine European humans strange indeed.
Here's a map of lactose intolerance around the world, by country.