When it comes to conventional warfare, Russian leaders have relied upon the same tactic for centuries: throw people at the problem. Russian territory has poor defensibility--so too do the lands of the countries Russia has between its own borders and most overland invasion routes. The solution? Plug the gaps with bodies. Lots of them.
What foreign forces didn't get mucked up in, well, the mire of the Northern European Plain, usually had to contend with the poor souls Russian leadership was able to place between itself and those forces seeking to invade. The advent of modern warfare has diminished the effectiveness of these tactics, from machine guns to modern artillery and drones, but Russian military leadership has to contend with another wrinkle: the mobility of modern Russian men. Gone are the days when you could corral villagers and locals and send them off to front lines. As we're seeing along all roads out of Russia, those who can run away from mobilization notices are choosing to do so. In the hundreds of thousands--to say nothing of those who are likely avoiding call up notices within Russia itself.