Machines have a lot higher initial one off costs.
The repair costs of some robots might make them more expensive per hour.
Humans are tried and tested, they don't break-down and stop your production line.
When designing the production lines they must weigh up the cost of a machine at each stage (initial cost, elec, repairs) vs workers and decide a time-frame they want to project and compare the costs over (because of their higher initial cost the longer period you do your projection over the more likely a robot will ever be cheaper).
Machines have a lot higher initial one off costs. The repair costs of some robots might make them more expensive per hour. Humans are tried and tested, they don't break-down and stop your production line.
When designing the production lines they must weigh up the cost of a machine at each stage (initial cost, elec, repairs) vs workers and decide a time-frame they want to project and compare the costs over (because of their higher initial cost the longer period you do your projection over the more likely a robot will ever be cheaper).