You have to remember that Western/European leaders risked a huge amount in convincing their public that supporting Ukraine was crucial. They told their public for months that they (the public) would have to endure heavy hardships and burdens in the form of not showering, not heating their house, suffering extremely high inflation, etc., all to “defeat Putin”. The public bought this because they were under the impression that Ukraine was winning. When you feel that victory is close, you will be able to bear such hardships because it feels worth it.

But as soon as victory no longer looks possible, your morale for bearing hardships will dwindle. European support to Ukraine is hinged on the European public believing that the war is winnable or that Ukraine is actively advancing and is “close to winning” in some way, shape, or form.

If Ukraine stopped attacking it would be construed as an admission of defeat, that they can no longer advance. Why would Europeans want to continue withstanding no heating, no showering, record high inflation and unemployment, etc., if there’s no further hope for victory?

So as you can see Ukraine is forced to continue sending waves of men to die just to appease their sponsors and make them believe that some form of progress is being made, all their hardships are not for naught.