You need infantry to hold urban population centres, it makes sense for russians to retreat and give battle in open terrain where they can deploy their mechanised and artillery forces with much clearer lines of sights, observation points and drones. Ukrainian terrain after all is flat steppes, ideal for artillery and mass mechanised formations. How are Ukrainians advancing in that terrain tho, they'd need large formations and numbers of IFVs, APCs and tanks to keep their infantry safe from shelling and small arms fire. 200,000 men is comically low for the vast steppe frontlines of Ukraine ngl, and not even all of them are frontline troops, war is manpower and industry intensive, slavic poons are not working as much now to replenish and increase their manpower.