Those markets are usually non-profit affairs and, may or may not have cut deals with resellers from the produce companies. If so, there may well be a load of California organic produce at a "farmers'" market most anywhere in the country.
... Otherwise, those vendors are people who often don't have a half acre for production. They can better be described as market gardeners. The US food industry became, long ago, a part of the US transportation system. That means a huge amount of energy is committed to moving food around this country. May as well mention fertilizer use and cold storage in that energy use.
I have visited California markets but it's been years. Often these places are local gathering places and food sales are almost secondary. But, people eat every day and leaving very much decision making to the industry doesn't seem like such a great idea for people concerned about a healthy diet for themselves and their families.
Be that as it may, my initial experience with California's farmers' markets was back in the '50's and they amounted to no more than 2 or 3 guys showing up in the Redding feed store parking lot and selling sweet corn and watermelons out of the back of their pickups. Washington State might have been said to be down 1 market when I moved north - Pike's Place.
People wanted something else. They wanted choices. Grow grapes, make your own wine! ... uh, raisins!
Steve