While we're at it, BMI is a not a particularly precise way of measuring overweight-ness. And yet, because it's easy, doctors stick to it, even knowing the flaws.

Which is why the ranges are so wide, and why no one would rely on it alone as an absolute indicator. But morbid obesity is highly correlated with bad outcomes, and most people with a BMI > 45 or who are 100+ pound over optimal weight are really prone to bad health issues.