>In which direction is causality flowing here, and how do you justify it? We didn't choose our brains

No, but our privileged parents did contribute to them, and their our upbringing, early months, lack of childhood stress over things as food and war, familial support, good schools, and so on.

Brains is hardly the best contributor to entrepreneurial success. In fact a plot of IQ vs success I've seen plotted looked like random noise except below a certain threshold (obviously seriously cognitive impaired people will have much more trouble becoming entrepreneurs). But aside from that, whether you are 105 or 120 or 150 didn't seem to make much of a difference.