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.
Also, we don't know the truth about their actual weaponry. Does Russia have those resources to sustain this war? And maybe, they are not using full force because they know the world is watching, and other countries may possibly help in case the situation calls for. Anyway, we don't know the reasoning behind this, and we are only giving speculations. What we can aim for is that hopefully this war will be over soon as many lives are now being sacrificed. The longer this war will be, the more sufferance it will give to both sides, whether Russian or Ukrainian.