Imagine you’re in Russia. You see every person who was ever a real threat to the government die or disappear. You can’t flee the country. A coup is highly unlikely; Prigozhin just tried it and the plane he was on mysteriously blew up. You don’t have a lot of options. Makes sense to take your chances against Ukraine than try to overthrow the Russian military/government.
What really needs to happen is for war law to be changed to allow heads of state and government buildings to be targeted. Then they’d be a lot more careful about going to war.