There is great incentive to get it right if your boss can come along and lop your head off if your work doesn’t come up to his standards.
What you propose was never a reality in ancient Egypt. Egyptian workers of that quality needed were not chattel slaves, like invented by the muslims thousands of years later, but more like a guild.
The heads cannot be carved with the hand tools accredited to them. And cannot be caved by hand to the finished smoothness of a modern machine - say a new car. Nor can the be carved by hand according to a mathematical exactness including mirroring.