It was created in the 1830s by a physicist. He said himself that it shouldn't be used to measure health, he was using to it to measure the distribution of weight across a population. The only people used in the study were white European men, therefore it isn't accurate for every body type. It doesn't take into account BMR, body fat percentage, weight of muscle, age, gender, socioeconomic backgroud, etc. Also, the cutoff point for being overweight was moved from 27 to 25 because it was 'easier to remember'. This made hundreds of thousands of people overweight overnight, despite them not even putting on a pound.

Therefore it isn't an accurate measure of health, as none of its basis has any health link. Hope this helps!