Investing in a known space

I recently finished reading The Millionaire Next Door. Super interesting overall and still relevant 20 years later. One topic, in particular, stands out. The authors found that wealth builders tended to invest in stocks related to their job. For example, doctors investing in drug companies.

This is a nice way of reusing specialized knowledge. However, because technology companies move so quickly, publicly traded firms tend to be in the mature stage. It's possible that the equivalent strategy in the tech industry might be angel investing.