CDC: The containment phase is really to give us more time. This virus will become a community virus at some point in time, this year or next year. [...] Right now we're in an aggressive containment mode, we don't know a lot about this virus, this virus is probably with us beyond this season, beyond this year, and I think eventually the virus will find a foothold and we will get community-based transmission.

Add asymptomatic transmission in an incubation period of ~14 days and there is not much to check. Also, since China waited months, the virus was already left unchecked, resulting in an estimation ~1 million infected: https://www.medrxiv.org/content/10.1101/2020.02.12.20022434v...