[throwaway22032]

If you're disciplined enough to put something in your calendar and do it over a period of months, without someone breathing down your neck to do so, whether you feel like doing it or not, then you are likely able to apply that effort in other areas of life.

So then it's a bidirectional correlation. You're more likely to be fit if you are wealthy and more likely to be wealthy if you are fit.

Essentially, what you're looking at is that people who engage in self improvement end up better off than those who don't.

It's a priori obvious but some people are uncomfortable with it for some reason - trauma response / coping mechanism, something like that.