Well, asking why someone didn't do something is kind of hard. Whether the Germans could have won is debatable, but what we can say is that they made a pretty good show of it.

With the final peace settlement with Russia in early 1918, that freed up a number of divisions to move west. The Germans were acutely aware that the Americans were going to be entering combat soon, and that this was their last chance before the Entente had the influx of men, with those already in France receiving additional training, and about 200,000 more arriving per month. France and the UK had dipped into the bottom of the well for manpower, an Germany had as well, exempting those in the East who they could now transfer over. So that spring, they launched Kaiserschlacht, a massive offensive meant to grab victory before the American forces could swing the balance solidly against them.

On the face of it, things started out pretty well, and they advanced deep into French territory. But just as with their initial offensive, the Marne was the place of their undoing, and at the Second Battle of the Marne they were stopped and forced back. It was their last chance. The toll had been intense, with the Germans taking ~500,000 casualties that they couldn't easily replace, while the losses taken by the Entente were much less pressing, as the Americans were there to pick up much of the slack as they began entering combat in the late spring/early summer.

With the Germans offensive stopped, and now utterly exhausted, the Entente launched their offensive - afterwards known as the Hundred Days Offensive, which quickly pushed Germany almost completely out of France by the time the Armistice was signed in November.