A Rolodex and a limited understanding of the real estate market. If they really had expert housing market knowledge would they be working as a real estate agent?
IME the ones who are successful long-term and make serious money do indeed use their market knowledge, connections, and access to make real estate plays themselves. Move into owning rentals, do some flipping, that kind of thing. The easiest path seems to be having a spouse who's in a different, but relevant, career (e.g. general contractor).
Sure, you can do all that yourself if you want, but for the average person, they are providing value.