1. They are doing it for propaganda purposes or
2. More likely, COVID poses a unique danger to China. For roughly 2 decades, pollution has been a major problem in China. Although recently air pollution has dropped to below what it was in 1990, it can cause the population to suffer from long term health affects so omicron could very well be as deadly to an average Chinese person as delta is to people elsewhere in the world. Here in the US, we saw something similar happen with abnormally high mortality rates in Native American communities because many people had chronic illnesses which resulted from toxic waste being dumped on the reservations.