Human operators are different, variable, and adapt

Human beings have characteristics that result from our biological makeup and which cannot be modified at will. They should be taken into account in the design of work systems, to the same extent as for the physico-chemical properties of products and processes. Otherwise the human being will adapt within certain limits, but always at a high personal cost and to the detriment of his performance.

Ref: human-and-organizational-factors-of-safety