One word: Resources.
Germany was resource-starved on just about everything.
- Iron ore had to be imported from Sweden (which expected to be payed for that -- and protecting those supplies was a big part of why Germany invested resources in invading and occupying Norway).
- Oil was in desperately short supply from the get-go.
- Huge amounts of coal went into synthetic fuel production to make up for the oil shortage as best they could (but yielding poor / low octane fuel).
Nickel, chrome, and other alloy materials that are required to produce high-grade steel. (The reason why the Me-262's Jumo 004 engines had such a short service live, for example, was that those alloy materials were unavailable by that point in the war.)
Rubber. Food. The list goes on. Germany relied on imports for all of these. So did the UK, but Britannia ruled the waves. So when England and France went to war over the invasion of Poland, German shipping was blockaded pretty effectively, while the Kriegsmarine -- for all its partial successes -- never managed to get close to the stranglehold on shipping that would have been required to even the odds on the resource front.
Russia, in the meantime, was sitting on a wealth of natural resources, by comparison -- and what they didn't have, they got from the USA by convoy, both across the Atlantic and across the Pacific (see above).
With the situation in natural resources being bleak and just getting worse with time, Germany's strategy revolved around winning quickly.
That hinged on the German army being as well-trained, and highly motivated, as it was. There was no point to plan for a long war, because Germany could not win a long war, especially not while Britain kept fighting. This meant that many long-term developments, especially troop and pilot training, were de-emphasized in favor of maximum impact right now.
So with every battle fought, with every soldier fallen, with every aircraft shot down, with every tank destroyed, Germany had to replace losses with less well-trained troops and pilots.
I cannot and will not judge whether the degree of training in the Red Army actually improved over the course of the war, or basically stayed on the same level. But it did not get worse the way it did for the Wehrmacht, that's for sure.
Result of Kursk.
Germany's goal was to cut off the Kursk salient, cutting off five Russian armies, and gaining the strategic initiative (as to eventually get to a point were they could, you know, "win the war" by forcing Russia into an armistice).
Russia's goal was to deny them that.
And while the Russian losses were high, they did deny the Wehrmacht every single of their strategic objectives. After the battle, the German Wehrmacht was exhausted, low on fuel and equipment, and utterly unable to conduct offensive operations.
The Russians were not.
What was once a crack, very much attacking force that had swept through France and Poland was no longer. Russia had the initiative, and thus the Battle of Kursk was a Russian victory, no matter the casualties.
You are comparing the wrong numbers, anyway.
So now, with all the above being said, let's look at your question, which exhibits "the usual misconception":
Casualties indicate the Soviets suffered 1 million casualties (MIA, KIA, WIA) while the Germans suffered 180K casualties. The Soviets tank losses were 10 times higher than the Germans.
Why were the Soviets so efficient at recruiting and replacing the lost man-power and material?
You make the error of comparing X losses against Y losses.
But what you have to compare is:
- X losses against X resupply, and
- Y losses against Y resupply.
Look at all what I said above.
Germany was resource-starved, blockaded, bombed, fighting on multiple fronts, and had lost its best bid at victory: A decisive blow, dealt on the move. It had been replacing crack troops with green troops rushed through training for quite some while, and was firmly on the defensive now. There was no respite in sight, and actually, the biggest tragedy is that it took them almost three more years and countless dead to finally fold to the inevitable.
Russia was resource-rich, convoy-supplied, its factories out of bomber range, and had been learning the lessons it had been taught by the Wehrmacht for two years now. Their losses were gruesome, but could be replaced. It does not matter if that is an "only just" or a "comfortably". The balances of attrition were now firmly in their favor, with all threat to their economical backbone removed.