Actually, closed borders make nearly zero sense once the virus is already inside a country. And even if it wasn’t you would basically have to isolate your whole country entirely for it to have any effect, no imports, no exports, nothing getting in or out, because if the virus enters in any way the whole concept is for nought. Sure, you might like that, but it would actually put the US into a severe economic depression and would probably lead to a major uproar by… well by people like you. And, well, the virus is already inside your country, so this is all just hypothetically speaking.
A virologist recently explained it like this: People entering a country with the virus add to the overall virus-infected population linearly. People already in a country are multiplying the virus-ladden population exponentially, meaning that the latter effect will always enormously overpower the former over time. Vaccines help against the exponential effect. Closing borders might at best help against the linear effect.
Closing borders after the virus is already widespread is nothing more than a political farce, a way to make people think of the problem as yet again something that was forced on them by the outside, not by incompetent and malicious politicians or their own ignorance.