The Apollo astronauts were also trained quite extensively in geology. Highly recommend the Apollo 12 episode of From the Earth to the Moon for a great look at that.
I guess as long as a high enough percentage of the crew are flight people, you can teach someone else to not get in the way too much?
Or it turns out that 20 years if field experience is harder to teach than 5 years experience in wearing pressure suits and zero/low gravity.