The fighting is taking place in eastern Ukraine, primarily along the Dnipro and the Donbas borders. There’s no sense in retreating because the Ukrainian objective at this point is to win outright, meaning they need to push Russian forces back to the pre-referendum border in the east. Ukrainian commanders are of course aware their supply lines are far longer than Russian lines, but they don’t have a choice in the matter. Any supply or repair depots within Ukrainian territory have been destroyed by Russian strikes. The only safe place for these support services is in Poland.

You have to get equipment to Poland somehow, and it’s not clear how Ukraine does it without functional rail. Meanwhile, even when this network functions, it’s not exactly efficient to ferry equipment all the way to Poland and back either. This is also costing Poland energy and resources. Poland doesn’t have much of either to spare at the moment.

Meanwhile, the status quo is hurting the west a lot more than Russia. Ukraine is collapsing internally, and it requires increasing amounts of support from the west to prop it up. Russia continues to dismantle the infrastructure in Ukraine, while Ukraine can’t meaningfully attack within Russia.

As I’ve said before, the real war is the economic war between the west and Russia, this goes far beyond Ukraine. So, having a stable line that Ukraine can’t advance past is giving Russia an upper hand over the west in the long run. There is no reason to throw away lives and equipment trying to do big offensives when you can just grind the enemy down through attrition.

They don’t necessarily need to be. The simple fact that it takes ages for damaged Ukrainian equipment to be transported from the front lines in the east to repair bases in Poland and back to the front lines again gives Russian forces an advantage. Meanwhile Russian forces have much shorter distances back to their support bases in Russia.