Why? "Overweight" and "normal weight" weren't determined by reference to what's better.

A big 30-year Danish study that seems to be one of the main ones referenced for this purpose sure appears to be doing exactly what I worried it was: measuring the predictive power of BMI for mortality, not the healthiest weight for someone to maintain, which are different things. I see nothing in the abstract & methods about correcting for sick people tending to lose weight, and the healthiest BMI creeping up over time as mean population BMI increased, plus the U-shaped mortality curve, really do look like what you’d expect to see if they’re not accounting for that.

Normal weight and overweight were determined by percentage of population at some point. The boundaries are not about lifespan, they never were.