Why look outside the system, if it is the system that is going wrong?

In order for the system to supply the need, the supply system itself, NEEDS the necessary People, Equipment, Processes etc. In order to be successful, these have to be of the right Quality, Cost etc.  Variance in the Critical Success Factors of the INPUTS (Quality, Quantity etc) to the supply system, are likely to cause the unacceptable variance in the OUTPUTS.