Because they are under the - in my opinion mistaken - belief that they will be able to deal with this using sanctions and external pressure alone. For some reason people seem to want to believe that the counterparty here is rational even if all of the evidence is against that.

It is very much like the run up to World War II, when countries were making all kinds of deals with Hitler regarding neutrality because they believed that that would keep them out of the firing line, when in fact it enabled a war on a much larger scale than would have ever materialized if the allied sphere had immediately struck back. But even the United States only responded after Pearl Harbor. So, now we have a real problem, and the people of the Ukraine get to choose between abandoning their country, fighting back or living under the Russian jackboot for as long as it takes to plunder their country.

This is not a good day, for anybody.