We tend to hit a Goodhart’s Law situation, where focusing on weight and BMI comes at the expense of actual health. That's how we get eating disorders and other mental health issues as well, so as usual it's complicated.
Assuming you are actually from the US, your point feels a lot like system justification[1] theory hard at work... If close to 50% (or even more) people around you have moderate to serious weight issues, I can imagine how you end up normalising the status quo, looking for explanations that rationalise it...
