I have two thoughts on this: One, they thought they could cover it up and there would be no global spread. Not because of a lab leak (though that is possible), but because of it damaging China's reputation. They lose their shit if you make fun of Jinping...
The other, they didn't recognize how dangerous and easy-to-spread the disease was. Remember, it is new. Yes, it is similar to other corona viruses, but the didn't see it as being a serious threat. And by the time they did, it was too late. It had spread to Europe and other places.
Either way, their mismanagement of this is why 26K Canadians are dead and we've endured these lockdowns and what not for nearly a year and a half.