More and more I think it's because it's difficult for companies to identify the capacity to be successful at this type of work early on. Partly because it's actually difficult, and partly because companies are really bad at identifying skills as a rule.
Instead they resort to the simplistic. Senior developers have made it through several filters. They've proven they can do the work, they've done it at several companies, and they've survived in the role enough to be promoted a couple times. That's just signal that they're less likely to run into problems, and that there's less risk the hire won't work out.
So, companies are lazy. Not intentionally—but because they're literally squeezing every ounce of time and focus on the very complex and wildly difficult task of managing a company and staying solvent and productive and going in the right direction.
They shouldn't be—they should have room for training and developing a talent pipeline—but this is the real world, and most companies can't even do basic company things really well, so the prospect of also doing on-the-job training really well to de-risk the hiring of junior engineers seems pretty far out.
In addition, many hiring managers have learned the hard way what '5 years of experience' means, because they've had 5 years of experience themselves, and only on the 5th year of that figured out how to do their job in the way they now know it needs to be done.
It's not an arbitrary set of skills they know they need, but an experience working in an ecosystem in specific ways that they know are crucial. Experience is not necessarily just skills, but a sequence of realizations, hardships, events, and successes that teach you things you can only learn by going through them.
Hiring managers hiring for a Senior role and pointing to a checklist of technical skills are probably not being fully forthcoming—they're likely looking for someone who does not see their own value as a checklist of technical skills.
Not saying that's you necessarily, and they could certainly be wrong, but personally I've learned that when the team needs someone with 5 years of experience, it takes 5 years to develop it, and there are no shortcuts to gaining that experience. The one thing I've found that speeds it up is experience in smaller companies or starting your own business—you'll experience a lot more much faster.
That said, there are companies that truly invest in on-the-job training, and with a holistic business model centered around that as a core value, it can be successful. It takes a philosophy, though, that most businesses will never mature enough to reach, dare I say, especially tech businesses.
The one I know of is the Greyston Bakery in NY, which was started by a Zen monk named Bernie Glassman (who passed away a couple weeks ago, sadly). His book "Instructions to the Cook" is really interesting, and outlines how they made a policy of hiring anyone who wanted a job work very well. They now run the Center for Open Hiring that helps other companies do the same thing.
It would be pretty incredible to see a tech company embrace that kind of thing and really invest in hiring and development as a strategy. I'd almost imagine it as a merger of something like General Assembly with an actual product and long-term business. It would be interesting to see if the significantly greater investment would be worth it over the traditional model.