Because it has already spread to other countries and so saying "No, there's nothing happening" would be implausible for everyone in the world. However, making up numbers to make it seem like something is going on, but is not as bad as it actually is, is entirely possible in an authoritarian state like China or North Korea. Not saying that's what they're doing, nobody knows, but it's really not unthinkable and it wouldn't be the first time.
Take SARS: it started in November, but they only mentioned something to the WHO in February and (worse) pushed down on press even reporting about it inside China.