Why is Russia using 'suicidal' zombie wave tactics on Ukraine frontline?

Ed Arnold, a European security expert at the Royal United Services Institute (RUSI) defence think tank, suggests the tactic of sending inexperienced fighters with little training in mass waves towards Ukrainian forces only results in a very slow, grinding attritional assault with no large-scale changes on the battlefield map. But Mr Arnold pointed out that where these kinds of assaults have gained some ground, often after months of fighting, Russia has lost that territory again in a matter of weeks because Ukrainian soldiers have managed to launch successful counter-attacks against the largely untrained force.