For example on wikipedia for Switzerland it says the country has an area of 41,285 km². Does this take into account that a lot of that area is actually angled at a steep inclination, thus the actual surface area is in effect larger than what you would expect when looking onto a map in satellite view?
I have seen farmers work on steeper slopes (not saying it would be easy…).
I would rather guess the purpose of these retaining walls is to slow down the soil erosion.