Explain Like I'm Five

The body mass tactic only works as long as those are able bodies. On defense were the oldest soldiers with lower quality equipment, with less motivation and ability to put up a fight. Hence the mass surrenders. Because all the young and easily compelled to fight were already on Ukraine. Now they will have to shuffle the forces, weakening the entire line and halting the recent offensives. An effective soldier can't fight in two fronts at once. Even if the progress is slow due to mass surrenders, you're still better off putting soldiers out of combat without losing territory (actually gainning it), instead of what was happening before.