I went to the farmers market on a weekly basis. I prefer the farmers market because unlike the supermarkets, the produce is fresher, it is more local, it is much cheaper, and I get to know who produces the food that I eat. I care about organic food, and unless I went to the farmer's market that was explicitly for organic food (though less cheap), talking to the farmers would help me understand how they farm and all the nuance around it.
Anyhow, this is obviously not more efficient than a trip to the supermarket: it takes more time and I had to go at a specific time and location. However, I don't go to the farmers market in pursuit of efficiency. I do so because I want to support small farmers in my area, I want to eat fresher, and I cherish a tighter relationship with the food that I eat and the people who produce it.
I do understand your point of view on efficiency is meant as a broader observation, but I find that it misses the points of farmers markets that are at odds with the pursuit of efficiency: localism, conviviality, and, give or take, qualities about the food itself.