Because they are weaker and less efficient than machines.
If I had the choice and can afford to choose between a person picking fruit and a machine picking fruit, I will pick the machine.
A human will get sick. A machine won’t.
A human needs breaks and insurance and have to be paid. Machines don’t.
If a human is broken, a life is affected. If a machine breaks down, it can be repaired and sent to the fields to work again.
Also, I can depreciate a machine as a deduction. When a human ‘depreciates’ for someone to profit from that, it is a crime against our own species.
I am a god in my farm because I own it. I don’t know any farmer who would pick unreliable, easily breakable, under paid(if they have a conscience, it should bother them) human worker over reliable machines because the prime directive of a business is to stay viable and profitable. The farmer wants to make a profit and hopefully in an ethical and sustainable manner.
Human workers for repetitive manual unskilled jobs is not sustainable.
Why can’t people be laborers regarding their basic needs some times, and academics sometimes?
It’s a pretty traditional values “I am king now and forever” notion to think one life one career.
There’s nothing biological preventing a rotation between work over our lives.
And many philosophers too have warned about division of labor making for a society of ignorant humans. Following orders rather than their curiosity.
IMO first worlders who can should. You’re not a god and the billions that outnumber us are catching up fast. Pointing big guns at them and saying “lol, no” isn’t gonna work forever.