[SahAssar]

> The reason seems to be: kids are cheaper than machines

I'd rephrase that as kids are cheaper to fix than machines. A burger flipper machine is almost definitely more efficient and cheaper per burger, but when it breaks its probably a lot more expensive to fix or replace than a kid.

It's the same idea, but I think that you need to separate the "cost per burger per hour per hour" and the "cost per burger per hour per year" since only one of those will consider the cost of unexpected costs.