That's not true. They could have standardized on a few rugged platforms -- and in fact, some in Nazi Germany advocated for that -- but their industry and engineering were generally self-sabotaging and a mess.
IMHO, the Soviets alone could have eventually defeated Germany, thought at much greater cost (as if over 20m casualties wasn't already incredible).
But the Nazis self-sabotaged constantly. The Panzer IV and the Stug III (with the outdated Panzer III chassis) were arguably the closest standard for armor, but they were constantly diverting effort to alternative platforms that were too complex to mass produce and maintain. And the same for other weapons.