Sorry, I think pretty much all those reasons are bogus. Yes, 100% agree that, as I said, it isn't a good metric for individuals, and I also agree that the actual calculation is weird - at the end of the day it's still just a ratio of height to weight.

But all of this nonsense nitpicking about bone density vs muscle vs fat is irrelevant when using it to look at large population cohorts over time. Looking at weight vs height is a good, easily calculable metric without needing to put everyone in a float tank.

The US BMI has skyrocketed over the past 50 years solely because we've gotten a ton fatter. There have also been plenty of corroborating studies that we've also gotten a ton weaker, e.g. studies that look at average grip strength.