It's "public" in the sense that most people on the Forbes list are there because they are either driven by ego and want to put out an estimate of their net worth for the rest of the world to see, or see it as an effective means of advertising themselves and their businesses. Both are interconnected and generally benefit the other. So for example you'll often see people like Donald Trump and Jeff Bezos on there because they really want to be there and/or run HUGE business ventures that routinely make the news, and being CEOs they want to burnish the public image of themselves as much as possible since they're the face of this or that corporation to potential investors and consumers. Add in the obvious competition to either make it to the very top of the list or even the top ten, and you've got an effective and engaging means of advertising certain executives to investors simply by virtue of who seems to be moving up the ladder the fastest and why. Average Joe's net worth is "private" because he doesn't own a huge multinational corporation that would benefit from showing him off in a trite and ego-pleasing rich person's equivalent of people magazine.