China makes almost all of the world's EVs, the world's motors and the world's wheels. Putting together any kind of agricultural vehicle is cheaper in China than elsewhere. That's just how it is. How do I know this? I just returned from seven years making robots in China. I will stop short of looking them up and pricing them, but feel free to do so.
AFAIK the Agricultural Bank of China is the largest bank in the world by some measures. It almost certainly has the most branches, and these are overwhelmingly located in agricultural towns and villages. Recall also, in communism sharing equipment is normal. People don't all need to buy their own. Chinese villagers like villagers elsewhere help one another.
Yes, flat areas exist. But they are not the test area, Yunnan. And if you were to look at the historical spread of rice farming (a subject of considerable academic debate) you would notice that all academic suggestions of the earliest invention of rice agriculture itself, with very few exceptions, appear to disperse through or via the test area, because it is the natural headwaters of rivers feeding the majority of mainland East, Southeast and (very close by) South Asia, including the Yangtse, the Pearl River, the Red River, the Mekong, the Salween, the Irrawaddy, and the Brahmaputra. Given we know that in ancient times river valleys formed natural communications paths this give us a fair case for the general dispersion of rice farming technology specifically through steep terrain areas and specifically through the test area.