I think the mindset for the Russians might be that the forces north and west of Kiev aren't adding anything to the fight just sitting there. In their mind they are willing to risk sending out those columns because it also means the Ukrainians also have to thin their lines in order to protect their flank, and if they get lucky and find a weak spot they will pour their troops there, and if they lose them than who cares.
These are the times that try men's souls. The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country; but he that stands it now, deserves the love and thanks of man and woman. Tyranny, like hell, is not easily conquered; yet we have this consolation with us, that the harder the conflict, the more glorious the triumph.-Thomas Paine