Oftentimes if the company is a USA startup, none of the experienced engineers have time to train or even mentor junior devs. A lot of startups want people who are "10x programmers" (however ridiculous that notion might be) or maybe it's less sarcastic to describe them as new hires that don't need any guidance or input, and will achieve maximum productivity after a short time adjusting to the new environment/toolset/workflow.

Also, you bring up an important point about time at the company. Anecdotal, but seems like a lot of devs spend 2-3 years at a company before moving on. That cycle is so short that the company doesn't think it's worth investing in junior people and doing so may even be considered counterproductive; the common view is that it's like investing in competitors. I'd like to think I'm wrong about that, but so far it remains a nagging impression.