Because we are led by donkeys. It was obvious very early on to the actual experts that this was not SARS. And if we had an actual plan involving experts we would be able to plot these differences and act appropriately even in more complex situations.
And while we didn't have that now so many world leaders have brought utter incompetence to the table. BoJo, Trump and that clown in Brazil spring to mind.
This is what I mean though. If your goal is total eradication within your country and then shutting yourself off from every country that hasn't done the same, then there are about 180 world leaders that are total donkeys. Even some countries that had initial superb success, even if relative, even if surrounded by total incompetence (Greece, Japan etc), are starting to see increases again.
Were many experts calling for a total national/international lockdown in late January/ early February? I don't recall them doing so to be honest. One of our own experts, for reference (Ferguson, who I do respect), has previously predicted potentially huge death figures for bird flu, swine flu and mad cow disease, as have the WHO. It is of course part of their job to look at potential worst case scenarios, though perhaps best to do so surreptitiously. Should we have shut down hard and fast for those diseases as well?
Johnson, Trump, Bolsonaro are obviously one end of the extreme (though the former is not the same as the latter two). But it is not these leaders that I am talking about. We're getting trapped in between the Brazil/ NZ approaches. Of course those are not the only 2 approaches and of course, there are almost 200 other countries/territories beyond the same 10 or so we often talk about in this thread, who have had to find their own ways to deal with it. My point is that, outside of some very small, very isolated island states (or Taiwan/Vatican City etc), there are almost no countries which have managed to achieve what you are asking for.
Which might suggest, putting aside the morality of all the approaches, that it is a bit more difficult than you think it might be.