I did not know that.
I did know Bill Gates' mom was on the United Way executive committee with IBM's CEO.
I think these details are often omitted because we like to promote the idea that all you need is hard work and talent and perhaps a bit of luck to succeed.
We tend to not like stories about success as a function of familiar ties. This may have something to do with our distaste for monarchies and aristocracies.
It's also a bit depressing when you're working on a startup but no one in your family is well off, and no one is on a board with the CEO of a huge and potential partner, and your brother did not sell his startup for hundreds of millions.