The only reason I have to hire a junior would be to develop them into a senior over 3-5 years, and companies are far too shortsided to think about that training pipeline.

Well and, without special arrangements, once trained for 3-5 years they'd now be more valuable and as easily scooped up by another company - if companies are short-sighted about the training pipeline, so too are employees short-sighted about how long they intend to remain somewhere