Because winning a major battle doesn’t necessarily mean that a war is won. The way a war ends is with the losing side surrendering, not when they lose a battle. It just gives the winning side advantage. For example, while the Axis powers won a bunch of battles and took over many countries in WW2, but that didn’t mean they won WWII. Same thing with the Allied powers. While they won something like D-Day for example, the war didn’t end there. It ended when Japan surrendered.