Because then we would always have to be the guardian, for all countries, for all time. We would become the evil empire for many people in the world (people who don't think the West should control everything), generating decades of future problems. We would forever be the spender of trillions of dollars on countless wars.
Plus, holding back seems to be working. Russia isn't doing well in the war, looking weak, and the world is behind Ukraine. Let the success be successful. Then, only if needed, squash Russia once the world is fully against Putin.