An interesting corollary: Why DID Germany surrender so easily? Russia was conquered, the British offensive in the Dardanelles was a disaster, Italy had almost collapsed and was more of an opportunity to the Central Powers than a threat. Even by the end of the war, the U.S. was only beginning to make itself directly felt. The new 'tanks' had not been effective at breaking the German defensive line. Militarily, Germany was losing ground, but the Allies had not even breached the Hindenburg Line.
Yet Germany accepted a humiliating one-sided peace! Why?
1. Starvation. Before the war, Germany imported a third of its food supply. The British blockade cut most of this off. By late 1918, the Second Reich couldn't keep even its soldiers adequately fed.
I remember once seeing a picture of the ration allowed to Germans toward the end of WWI. It looked like quite a bit of food, enough to feed a person for a couple of days, with a whole loaf of bread, bits of sausage and butter and jelly even. Then I read that this ration was not for one person, but for a family. Further, it was not for a day, but for a week. The ration could not have been life-sustaining for any long period.
Which leads us to:
2. Revolution. What actually precipitated the German surrender was not defeat on the field of battle or even magical Americans, it was Germans who couldn't take any more. Even before the Army mutinies started, German soldiers were surrendering in droves. Communists and Socialists helped instigate riots in the cities.
Karl Marx said that no country is ever more than three meals away from revolution.
Hitler was not entirely fantasizing when he blamed the defeat on the 'fifth column'. Germany was ripe for rebellion, and in those days the Leftists were ready and able to encourage revolt. They even briefly set up a Marxist state in Bavaria after the war, and if the Communists had been willing to come to terms with the Socialists, the fabled Proletarian Revolution might really have happened.
If the Allies had wanted to defeat Germany quickly, they could have encouraged and supported Germany's domestic 'enemies'. They did not, perhaps for fear of vulnerability to their own working classes.