system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are so many people in the world unable to digest lactose?
gpt
Evolutionary pressure. Once you're weaned off of your mother's milk, there's really not a lot of need for dairy products. These products are very recent in human history, as we didn't really have access to milk, cheese, etc. before the domestication of cows. From a biological perspective, it wouldn't have made sense to keep producing lactase (the enzyme that digests lactose) if you weren't eating anything WITH lactose.

You can see this influence based on lactose intolerance by region. Europe has significantly lower rates of lactose intolerance, most likely due to earlier attempts at widespread dairy farming. Evolution would dictate that having the ability to digest milk into adulthood would have an evolutionary benefit in that situation.