There’s a lot that governments can do. Ireland just closed schools and universities, and banned large indoor gatherings, for instance, and directed companies to make work from home available where possible (though this last one is really more a suggestion). This won’t get rid of the virus, but it may help to slow it and thus reduce deaths.
Closing borders, certainly at this point, seems largely futile.
I've read (true, who knows?) that in the USA this isn't legally possible. Federal government doesn't have that power.
This may be behind Trump's move to stop flights only to Europe, and not domestic flights. He has the power to do the former but not the latter, apparently.
It has everything to do with deflection, point out that the problem is over there, even though the virus is spreading inside already unchecked. The Governor of Ohio is putting an order to shut down all mass gatherings like sporting events and also suggesting people stay at home. Ohio State was closed before any cases where confirmed but it doesn't seem like other states are taking it as serious outside of California and Washington and testing is still only happening for cases that are in intensive care or had traveled still. So we have no idea what the true lay of the land is at this point.
If the internment of Japanese-American citizens during WW2 [1] was legally possible for the federal government, to me it would seem odd if cancelling Coachella wasn't.
The US courts have since explicitly rejected the reasoning used to justify the internment of Japanese-American citizens. It would not be legally possible today.
Closing borders, certainly at this point, seems largely futile.