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 school answer was apprenticeship - a company trains you up, and in exchange you work for them for a time. But most companies are loathe to do that because you're likely to quit and go somewhere else for a pay bump as soon as you have enough skills and experience that you're worth hiring.
For my money this, right here, is the real problem with modern programming / career development. Whiteboard interviews are the least of our problems.
This is only true in bad companies. Good companies understand that people move on, and don't assume that someone is hired in to a role forever. Once you realise that hiring is an expensive process that you will always be doing you then you can optimize it appropriately.
Some really good companies even use it as a point in job adverts. There are plenty of senior developers who actively want to teach and mentor juniors, and will be happier working in companies that encourage that process. It's a good way of retaining those staff.
Yes, they understand this, so they simply do not hire people who need to be trained for a year or two to be effective hires.
One problem is that companies with the means to fund this- FAANG and the like- are also the ones are incentivized to gatekeep the most to maintain their elite status and engineering superiority that leads to business edge. Startups, being naturally less risk adverse, are less likely to engage in formalized apprenticeship programs for juniors.
What happens instead is that you get the half-hearted approach of hiring college students to be interns or co-ops. Not all students are able to intern. The ones who do have a leg up once they graduate. Hence the stellar reputation of U of Waterloo grads.
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.
“Why would we train them up, they’ll just leave anyway” -> “screw this place, I am going to leave when I get the chance” -> leaves -> go to step 1
