It doesn't have to be that way:
It may not have to be that way; my point is rather that it often is that way, and it's not out of incompetence. An army that expects to expand dramatically in war time still has an interest in preserving an officer cadre, even if they have no troops to command. In this respect, senior, experienced NCO's are arguably even more important than the officer class themselves, as in practice green officers are often better off letting veteran NCO's be the de facto unit leader until they have some experience of their own under their belts. On top of this, the Junker officer class had a military tradition dating back to Prussian times which they proudly maintained and defended, and it may well have been politically impossible to dispose of them even if you wanted to.
This reminds me, incidentally, that it was argued above that the Ardenne assault was a demonstration of German military competence; however, I suggest this may be the wrong way to look at it. Because not only had the Germans invaded France through the Ardenne at the beginning of the war, but they had also done so in the First World War. So in fact the Battle of the Bulge was the third time that Germany had used the Ardenne route, and it can be argued that the surprise they achieved was really a failure of imagination on the part of the Allies to assume it couldn't be done, although admittedly neither of the previous operations had been carried out in winter.
If you are really savvy you select your Officers on the basis of their loyalty to the Government rather than their competence, perhaps unfortunately that wasn't an option open to the Weimar Republic.
Hmm, not sure many would agree with that. I think the issue of how the Nazis came to take over Germany is a rather separate one; I think we kind of forget just how
weird Nazi Germany really was. The other day I was looking at pic of a grenadiers helmet bearing the German flag on one side and the party symbol on the other; can anyone today really imagine how strange things would have to get for say the US military to be sporting the party symbol of the Democrats or the Republicans?