I think part of it is that the sanctions take time to work. The impacts are not felt instantaneously. Particularly if you're looking at currency reserves, as I understand it, the Russian government still has some of its reserves at its disposal.
Of course, there's the reality that not every country has placed sanctions on Russia. Unfortunately, there are a lot of large markets that have not gone down that route. Canada, NATO and the western alliance have done it, but as long as you have countries willing to do business with Russia and not place sanctions, there will be an additional funding route for the Putin government.