When factories restart, not all of the workers who were working there before actually come back to work. Some of them stay in their hometowns or they get other jobs. This means that restarting the production line actually means retraining people on how to assemble the product. There is also an entire supply chain behind the assembly line that takes time to restart. Think of all the sub-components like plastics, metal components, etc. that need to be built at respective factories. It takes time to ship them to the primary factory for final assembly and test.
After the product gets assembled, there are several stages of testing, like gluing, environmental testing, final assembly test, and packaging, that take time as well. Then the product has to be shipped to the fulfillment center, packaged, labeled, and then shipped out. Each step time, and the process needs to completely re-start after CNY.