> Everyone knows BMI is imperfect at this point, ...

Indeed.

BMI is the best thing that people can readily calculate with easily available equipment (a tape measure and scales either at home, gym, pharmacy, etc) plus some relatively basic maths or sticking the numbers into a website.

Measuring body fat using calipers is better but hugely error prone. Similar for waist/height ratios. Body fat scales can be wildly inaccurate.

BVI is far superior but very few people have access to the equipment needed to measure that.

So we're kind of stuck with BMI as the best "simple" measure.