It wasn't. Countries like Italy and Germany believed China, and that alone would probably have been enough to doom efforts in the countries that didn't. Also, as the other reply says, patient zero was likely already in the US, so any effort at successfully containing this would have required rounding up and quarantining people from Hubei already in the USA and their close contacts back when the press were still downplaying it. (That's effectively what China did, though it was... a little ugly, shall we say.)

Italy was one of the first countries to ban flights from a China.

Patient Zero was already in the United States.

There are 100,000 flights daily around the world.

Even after the US had a travel ban with China, flights were still leaving China headed for America.