There are existential questions that gnaw at some of these people that hinder them from taking the seemingly simple path that you have presented.
I would also argue that it is not as easy as you would think to park millions in some "low interest stable investment" that also does not have substantial (for generational wealth) counter-party risk.
> there is no equivalence between the road the rich and the poor need to walk to survive
I agree with this, but I think you are being overly dismissive of the skills that someone born into a wealthy family needs to keep that wealth.
It requires a healthy amount of self-awareness among other things.
That doesn't change the fact that being rich is easy mode for life. Yes, it isn't just a stroll in the continuous orgy park of happiness and funtimes, but there are a lot of difficulties in life that arise out of not having enough money, rich people essentially get a pass on all of these problems.
So, while I'm not saying that rich people never struggle with being a human being and all the existential doubt that can accompany that. I am saying that they never struggle to eat. I think everyone has their own struggles through life and really don't want to minimize problems that can arise in dealing with human issues like poor mental health - but seriously, everything is on easy mode for the rich, and if you don't think that's the case then you should really reach out to other communities and see what low income people have to deal with on a daily basis.
I think another perspective on this issue is worth making.
Rich people literally just need to not be complete idiots - if you've got 300 million in the bank put it in some low interest stable investment and you'd be able to live off the investment earnings alone. There, I just explained all the life skills you need as a rich person - there is no equivalence between the road the rich and the poor need to walk to survive.