The absence of lactase, or its reduced activity, means that the lactose doesn’t get digested in the small intestine, so it passes into the colon, where it encounters bacteria that can break it down and it ferments, causing gas buildup. That’s the direct cause of the bloating and fartiness, but also the increased pressure triggers diarrhea, and so on. This is called lactose intolerance, and admittedly though not particularly pleasant, it’s not the worst condition someone can have, and is pretty normal for most people if they drink milk into adulthood. Which is why most people don’t.
