Because then rich capital-holders could buy up all the computing power and take control of all the welfare money. Chinese millionaires would end up taking money from needy Americans.
Using this system, we ensure that no single capital-holder controls too much mining power over the currency. Every working man is judged on his merits, using a system that is hard for any one actor to parallelize or scale. So many many distinct actors work to secure the underlying currency, and it is safer against large banks or governments.
Except that if I tell you I have "a million people" in my secret facility manually hashing and in reality there's a single GPU crunching, I'd still make more. How would you audit this?
This is silly. One small mining computer would be faster than all the human hashers combined. There is no special hashing function a human can always do better than a computer.
Using this system, we ensure that no single capital-holder controls too much mining power over the currency. Every working man is judged on his merits, using a system that is hard for any one actor to parallelize or scale. So many many distinct actors work to secure the underlying currency, and it is safer against large banks or governments.