You can see why, though. The Spartan-IIs were kidnapped as children as raised military from then on. The Spartan-IIIs were teenagers orphaned by the covenant who were willing to be augmented for the purposes of dying in a suicide mission. Spartan-IVs were normal soldiers before their augmentation.
You see the technological progress. The augmentation process has grown cheaper and the age at which it can be performed has grown older, which is why there can now be an army of them. They're limited now only by the number of recruits who make the grade and the cost of the GEN2 MJOLNIR, which is still orders of magnitude cheaper than the Mark V.
The advances do mean that each generation of Spartan is less capable physically and less well trained, but the numbers and cost-effectiveness more than make up for it.
Also, surviving members of previous Spartan programs were invited to join the Spartan-IV program, so a few of them could be IIIs, or even Orions.
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