Comment by justapassenger

One of the reasons that I saw, is that training is very expensive for the company (both in terms of money, opportunity cost, time of other people, etc) which combined with people changing jobs often produces low, or negative ROI. By the time person is trained, they leave, before they start to meaningfully contribute. More experienced folks also change jobs often, but you have higher chance of getting some productive time out of them before they move.