Same for me, no one considers BMI to be a good indicator of anything, but it is better than nothing, and more importantly, it is super simple.
Any idiot with a bathroom scale and a measuring tape can do it. And by adulthood, height doesn't change significantly and you probably know it, so you may not even need that measuring tape. Other metrics need specialized equipment, error prone or complex procedures, etc... I don't even know how to make a waist measurement. Where exactly? How relaxed should the subject be? How long after eating?...
That's the value of BMI: simple repeatable. Not the best but enough to get and idea and make statistics.
> And by adulthood, height doesn't change significantly and you probably know it ...
But height does change in adulthood [0]. On average people living past 40 gradually lose height. At age 80, it's likely height loss will be >=2 inches. This loss is reflected in higher BMI even when body fat content hasn't changed. Average BMI increase isn't large, but think about a person losing 4 or 5 inches of height while maintaining body fat unchanged.
Self-reported height is generally greater than measured height. Adding 1/2 to 1 inch is common (per experience measuring height). "Height inflation" has modest effect on BMI but problematic in research and workup for medical procedures. In the latter cases, measuring height is necessary.
[0] https://www.uhhospitals.org/blog/articles/2024/10/why-do-peo...