> Is it not entirely logical that a cohort that actually pays attention to where 20%, 30%, 40% of their money is going ends up wealthier than one that doesn't?

Many if not most rich never got rich, they happened to be born rich. In the situation I'm talking about, the said family has been wealthy for two centuries. And my in-laws are significantly less well-off than the generation before. Their ancestors may have been gifted in some way that got them rich in the first place, but there have definitely be some regression to average since then and their management of their wealth has been much less efficient than the one of my parents for instance (who aren't wealthy today, but are still clearly better than their own parents).

In fact I suspect that these people are obsessed about taxes because they know their fortune is ever going downhill because they lack the thing that made their ancestors successful so they spend their efforts trying to salvage their wealth instead of making one of their own.