In a sense, it's an example of the tragedy of the commons at a national level:<p><a href="https://en.wikipedia.org/wiki/Tragedy_of_the_commons" rel="nofollow">https://en.wikipedia.org/wiki/Tragedy_of_the_commons</a><p>No one nation feels it should bear the burden of doing this alone, and so none is especially motivated to do so.<p>To be clear, even if all nations were united under a single government, that government would likely feel little motivation too, owing to the perceived low probability of an asteroid event in any given political time period.