Earlier quoted context omitted.
I don't know! Is it their job? The attention of senior engineers is the single most valuable, expensive and scarce commodity of a modern software company. The incentives are absolutely not aligned for most companies to make it worth their time to hire junior engineers. But obviously thats a problem - because as you say, where else will senior engineers come from? And I don't think we have a good answer here. The old…
There is the part where you give junior pay raise after he or she learns enough to get raise elsewhere.
But there are bigger problems. I've experimented with training people in various ways over the years.
Reality is programmers move around a lot. They are attracted by interesting new problems where they feel they're learning. Staying at one firm for 20 years isn't likely these days. There's nothing wrong with that, but it means if you spend a few years training someone then after that time period they may leave anyway, even if their comp is reset to be competitive, because the new place can offer them equal comp + new problems.
Another issue is that a lot of training junior-to-senior is about imparting experience, wisdom, beliefs etc. At some point they can code and it's about the decisions being made, rather than inability to do them. A characteristic of junior devs that are growing their skills is they tend to latch on to trends harder and quicker than senior people who have maybe seen it before, and can differentiate their CV without buzzwords. If a junior comes to work one day and says "We need to Kubernetize all our things" and you say "Actually, our server count is stable and low, we don't need to use Kubernetes, but we do need this new customer-facing feature implemented" then it's quite possible they'll get frustrated, want to argue with you. Of course replace Kubernetes with Haskell, Linux distro of the day, Rust, Go, whatever seems hip and new.
It can just end up being draining for everyone. Of course debating these issues can be teaching of a form, but often juniors don't see it that way. The student wants to become the master faster than sometimes makes sense.