Because any country fighting a war on defense against aggressors/invaders knows that the path to winning is to hold strong, wait out the clock, and drain resources from the enemy. Something as essential as that will cause urgency to redirect resources and personnel to replace/repair and then better defend it so that it doesn’t just immediately happen again. This makes defense easier because there’s less of an offense while that’s happening. This also can cause internal loss of support since Russian citizens can see the costs of this going up. They will know people who were sent to die on the front lines. They will see taxes go up and availability of goods go down. Once Russian citizens start to question and criticize the campaign, there could be a snowball effect that ends with Putin and his cronies having to make a choice between stopping the invasion or losing power. I don’t think Putin will ever stop, so the real choice will be desperate attacks (which could include nukes, triggering article 5 and effectively ending Russia) or a coup. Putin has checkmated himself whether he is aware of it yet or not. His best case scenario at this point is dying of natural causes in office and leaving that hard decision to his successor who will probably back down and be a pariah for it, saving Russia in the process.