For some, you get the Jack Walsh thinking that some employees are going to be statistically bad performers, so it is good to get rid of them.
You also have other cases where lowering the time to train means you can expand faster since you don't need to find quality staff. The original McDonald's trained its staff to be able to be high output restaurants. The business model changed to needing less worker training to help fuel expansion.
You also have the case where some managers believe some jobs only require a commodity level labor. At that point, there is no value in training.