There are several possible reasons for why it didn't happen. The first of which is that contrary to what you state here the Nazis never really prepared for a large scale insurgency in the event of a defeat. Such preparations were considered defeatist and generally not encouraged. The one serious preparation that was done was the Werwolf project, however that was never intended to be a guerilla movement, it was intended to be commando units fighting in uniform behind enemy lines.

The Werwolf project never really took off and their impact as insurgents was almost non-existent. They lacked manpower, equipment and organization to function effectively and large parts of the Werwolfs were absorbed either by the SS or the Volkssturm. Fritz wrote a bit about them in Endkampf: Soldiers, Civilians, and the Death of the Third Reich, as did Biddiscombe in The Last Nazis but it seems much of what remained after the SS and Volkssturm had taken their pick was incorporated in the Ratlines which helped Nazis officials escape.

EDIT: Occupying allied forces generally described germans as cooperative and friendly.

In the end even the hardest core nazi would have seen it as pointless. Hitler was dead, the Wehrmacht was shattered, the country was in ruins and occupied by millions of allied soldiers. Most importantly the majority of those soldiers were pretty benign. The Americans immediately poured money in via the Marshall plan. Also in those days we had declarations of war and unconditional surrenders.