Likely because most of the year they are laying groundwork for when the large amounts need to be set up, working out kinks with small batches. The end stuff is likely mostly done by seasonal employees, which often come in from the collective and private farms to the city to work (depending on the region) during the off-season.

As well, China has pretty high unemployment rn, especially among the youth, so they can marshall these kinds of labor resources whenever they really want, it is just a matter of the state actually wanting to accelerate whatever 5-year plan targets they are trying to hit.

I'd assume making sure they meet whatever goals are set.

Yeah probably rushing to hit targets I guess