Probably because they are underestimating Ukraine and the tenacity of the Ukrainian people. But I don't think they would underestimate it to the point that it would make the war drag on this long. One thing I can think of would be the way they think that they should always be prepared for whatever implications their invasion on Ukraine may bring. Although it may be unlikely for the other countries to be involved to an extent of attacking Russia directly but I guess they just want to be well prepared for whatever scenario.