[wazoox]

Do you know why work is so much more productive? Because so much is automated away, and relies on machines. Each and every worker works using lots of machines. Of course, the more machines you can make work the more productivity you have.

When a large part of your work can't be delegated to machines, for instance nursing a baby, talking to people face to face, etc (mostly "human relations" things) then your productivity can't augment as much. This is also why "human work" earns less than "things work".

Do you know what machines use to work? Energy. In the end, energy availability is the ultimate constraint. Good old thermodynamics. Why can't anything grow forever (and surely not GDP)? because of available energy, and thermodynamics.