Because Putin's primary purpose is to maintain his power. Even at the expense of the happiness of the whole Russians.
Dictators have more to lose than politicians in democratic countries when they are overthrown. So smart dictators want the world to be as stable as possible. A typical example is the Kim family in North Korea. While developing nuclear weapons , they have surprisingly avoided military friction for the past half century.
Clearly, war is a good way to gain support of citizen in a short period of time. Putin did the same when he annexed Crimea and Donbas. But if they become addicted to it, the only way is ruin. Once a war has been started, a dictator must wage war repeatedly to maintain power.
Remember what Hitler did. At that time, Germany was suffering from the Great Depression. Nazi's annexation of Austria and Czechoslovakia enthralled the people and gained support for a while, but it does not solve the problem in essence. So they invaded Poland and committed suicide six years later in a bunker in Berlin.
Anyway, what I'm trying to say is that war is too risky for Putin. Total victory in modern warfare is not just winning the battle, but you need see the big picture. At some point, the brakes have to be applied, but it is difficult to expect such a thing from a dictator.
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