Because they won territory. If you win territory in a dispute, it's much easier to sell the whole conflict as a win to your population. If you come home empty-handed, it's much more difficult.
Tell me: if the war concludes and Russia gets territory, why wouldn't they do it again? Presumably the reasons for the initial attack still exist. If Russia sees that their previous invasion netted them territory, how is that not a confirmation that they can win territory by invading other countries?