We managed to reduce...

Not quite reduce. We managed to develop certain approximations which, albeit crude, work sufficiently well for some purposes. Of course, not all purposes.

I seem to recall that BMI and/or WHR tend to be excellent predictors of physical health.

I seem to recall they tend not. In particular, BMI is a flawed indicator as it has a pronounced bias for short and tall people.

these predictions can further be improved

Which "these predictions" -- what are you forecasting?