This is no less true for the average person. If the real estate market in your area crashes and you own a house, your net worth would crash through other people

This is no less true for the average person. If the real estate market in your area crashes and you own a house, your net worth would crash through other peoples collective action. The only difference is that most of these billionaires are invested in a few specific companies, however there are plenty of people who (likely for bad reason) are also heavily invested in only a few companies.