The main reason for this war dragging on so long is that the actors financing and driving this war aren't using their own constituents/people to fight it out. In other words, western powers get to throw Ukrainian slaves (which are willingly sold by Zelensky and his goons) into the fire without having to fear any negative consequences at home short-term and long-term. Normally, there is a certain threshold for loosing manpower which forces elites to concede when a war is lost. The situation in Ukraine allows western elites to use up far more manpower than usual because they don't care about the long-term outcome for the "Ukrainian nation".