I think it's because most of the German propaganda photos are of mechanised units, not horse-drawn ones, as the Nazis wanted to show their own people (as well as the rest of the world,) that the Wehrmacht was a modern fighting force. Eighty years later we are still feeling the effects of Nazi propaganda
I first learned about the German Army's lack of mechanical transport from reading Len Deighton's Blitzkreig in the 1980s. (Published in 1979.) I was astonished that German industry couldn't produce trucks in sufficient quality or quantity for the Army, and that even captured enemy vehicles and the production from occupied countries didn't make a dent in the needs of the Germans.
Whatever, you can troll away but if you actually interested in learning more about the rise of Hitler I highly recommend these two books (which were actually written quite some time ago - but both have aged very well).
This is an easy and pretty quick read and super interesting beyond just the rise of Hitler. It documents extensively why/how Case Yellow (the invasion of France and the low countries) worked.
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https://www.amazon.com/Blitzkrieg-Rise-Hitler-Fall-Dunkirk/dp/0007531192
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This is probably the best book I've ever read on the Third Reich and it's super famous for a reason. It was published in 1960 and is still absolutely fantastic and honestly should be required reading:
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http://www.bbc.co.uk/history/worldwars/wwtwo/ww2_summary_01.shtml
There are some books listed at the bottom of this timeline. Blitzkrieg: From the Rise of Hitler to the Fall of Dunkirk by Len Deighton seems to cover the European theater in good detail.