They believe that a pre-trained pilot will actually take extra effort because they will have learned habits that are inconsistent with the training on offer.
Personally, I believe it's nonsense.
But, from another point of view, they might think you're an idiot for spending all the money to get a CPL and yet be willing to spend it all again for the same training albeit with a job at the end. And, in that, I'd tend to agree.