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.
"Not having the time" to introduce new engineers to the problem at hand, task, system or what ever is a recipe for wasting even more time in the future. It's very short sighted. Neither "10x programmers" or mere mortals can grasp a code base by reading it alone as by a thorough introduction.
I mean, a mediocre engineer that knows the system well is way more useful than an excellent engineers that just got his hands on it, for like half a year or more depending on complexity. If you give guidance you don't need excellent engineers. At least not as many. It's kinda surprising the way companies think of this.
Rather than officially allocating a developers time for introducing new employees or writing documentation for the system, Company 101 is to put 10 or 100 times the amount of dev time into catching up for the devs replacement when he quits, after he quits.
I agree wholeheartedly that it's shortsighted. Junior devs need training and guidance from experienced engineers. I view the problem as a failure to take the long view of progress.
At the same time, these startups find it difficult to hire people especially when they have money but hiring is expensive in terms of time for a small startup team. This must be changed for the benefit of all involved - job seekers, company.
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.