Because supply chain logistics. The Germans in WWII were world leaders in manufacturing (perhaps bested only by the US), and one of the elements of that manufacturing quality is the ability to trace individual parts back to the exact manufacturing batch to figure out why particular batches go wrong.
The Germans did (eventually) make some effort to obscure details of their supply chain--they forced manufacturers to use three-letter codes instead of their normal trademarks--but that still suffered from poor operational security which allowed the codes to be quickly matched up to manufacturers. It didn't help that the British analysts meticulously kept track of everything, allowing them to identify the manufacturer of one unlabelled part by the inspector's number.
They might have had good equipment sometimes, but they had nowhere near enough of it. They were outproduced by Britain “alone” (counting the colonies) in most areas, most the time, and often by considerable margins.
The German army was not particularly mechanised or well equipped as a whole, relying on a lot of horse draw vehicles for the entire war.
When you look at the war from a manufacturing perspective, the question is more about how Germany survived for so long again it’s such huge manufacturing nations.
For a seemingly dry subject, David Edgerton’s book on this is very readable. https://www.theguardian.com/books/2011/mar/27/britains-war-m...
I should say that Germany was a leader in manufacturing quality, not so much quantity, although I believe they did comparatively well there considering that they were facing down the gargantuan industrialized economies of the US, UK, and USSR.
It's also worth point out that Germany suffered from a severe lack of resources, particularly oil and rubber (although everyone in WWII was short on rubber). While they did have synthetic fuel and rubber plants that they made excellent use of (part of the reason for German superiority in the chemical industry was their need for it), these synthetic routes are not really sustainable for a massive war effort, and Germany ran out of their stockpiled reserves by 1942. Case Blue, the second offensive in the USSR, had obtaining the Baku oil fields as its main objective.
Even quality I'm not so sure. Because quality largely depends on the intended use. And if that use is to shoot at things an be shot at in return in abysmal operating conditions the traditional German quality standard is just over the top and unsuited. And while quality beets quantity on a per unit basis, globally there is inly so much quantity difference quality can make up. Something Germans seem incapable of culturally understanding even today the concept of "good enough" is beyond the understanding for a lot of my fellow country men.
German quality was largely a myth. Examine tanks; they need to be survivable, reliable, and potent. Without all three, they're useless. German tanks were rarely the best on the battlefield when measured this way.
Aircraft (especially fighters) have the same three requirements: until the ME-262 was deployed, Germany was only on par with the allies.
Artillery? Other than the feared 88mm, its artillery was clearly second fiddle to the Allies.
What enabled Germany to have any success was the initial training of its NCOs and officer corp. This allowed them to exploit opportunities faster than their opponents (think of Boyd's OODA cycle).
But all the oft-touted German "super-weapons" were usually over-engineered stuff that didn't work reliably. Note that the ME-109 flew until the end of the war since it was reliable, and effective against bombers until they were escorted by Mustangs and Thunderbolts.
It wasn't about lack of industrial know how, it was the policy of Autarky. The Nazi regime literally starved itself of essential manufacturing and war time resources.
They knew for example that they only had enough oil, with the limited mechanized forces they had, for operational effectiveness until autumn 1941. After that Germany would never again have the resources for grand operations on the strategic level of operation barbarossa. They needed to get to the oil fields of the Caucasus region which they did not even get close to due to some screwed up leadership decisions.
Fall blau was a pale comparison to the earlier operations and Germany's logistics system and resources were beyond tipping point.
And then when it came to Kursk all they could really manage was a single limited scope battle.
The Germans did (eventually) make some effort to obscure details of their supply chain--they forced manufacturers to use three-letter codes instead of their normal trademarks--but that still suffered from poor operational security which allowed the codes to be quickly matched up to manufacturers. It didn't help that the British analysts meticulously kept track of everything, allowing them to identify the manufacturer of one unlabelled part by the inspector's number.