It seems as though it was defeated twice and nobody took the entirety of the land. Why was that? Seems like an easy cause to support at the time given the amount of conquering Germany was doing.
First off, Allied armies occupied Germany after the Second World War for some near fifty years. In the immediate aftermath of hostilities, the administration of Germany was through four military governments. Germany also lost significant parts of its pre-1933 territory in the east to Poland and the USSR. The German population in these occupied areas largely were expelled from their homes in these eastern territories. The Soviets and others justified this forced expulsion of German population, as well as ethnic Germans in other parts of Eastern Europe precisely because of the behavior of the Third Reich. Although both the western and eastern zones grew into two independent German states, in many ways their sovereignty was compromised by the nature of the Cold War in which their respective superpower patron made demands to base massive military forces on German soil.
As for the First World War, the lack of total occupation of the German state was largely because the new German government that came to power in November 1918 agreed to an armistice while Germany proper was not occupied. While there were some voices in the Allied camp, most notably American General John Pershing, that called for the remaining German armies in France and Belgium to be pushed back to Germany, theses voices were in the minority. Aside from the obvious moral questions that come from continuing a war after an enemy sued for peace, arguments like Pershing seemed moot since the one of the key clauses of the Armistice was the swift evacuation of German troops past the Rhine River. Allied forces occupied the Rhineland bypassing Germany's network of defenses and fortresses at little cost. As with the Second World War, Germany lost territory at its periphery.
The situation of the expellees after the Second World War illustrates a larger point implicit within this question. While it is one thing to take land through a political process after a military conquest, this leaves the problem of what to do with its existing population. The world is not a game of Risk and populations had grown so much by the twentieth century that expelling them was no easy matter. The Third Reich's solution to this dilemma was a policy of systematic genocide. Mass murder of the entire German population was something that was politically unacceptable for the victorious Allies in 1918 and 1945. While German populations in these border areas would be displaced and forcibly transferred to the German core, doing that to the whole population of Germany was too extreme a policy.
